INTRODUCTION  TO 

MODERN  SCIENTIFIC  CHEMISTRY 


m  i 


BY 


PROF.  LASSAR-COHN. 


J 


LIBRARY 

OF  THB 


UNIVERSITY  OF  CALIFORNIA. 


'\    \    Class 

I 


INTRODUCTION    TO 
MODERN     CHEMISTRY 


AN   INTRODUCTION   TO 

MODERN  SCIENTIFIC  CHEMISTRY 

IN  THE  FORM  OF  POPULAR 

LECTURES   SUITED    FOR    UNIVERSITY 

EXTENSION   STUDENTS   AND    GENERAL    READERS 


DR.    LASSAR-COHN 
u 

Professor  in  the  University  of  Konigsberg 

Author  of"  Chemistry  in  Daily  Life,"  "  A  Laboratory  Manual  of  Organic  Chemistry ' 
and  Hon.  Member  of  the  Society  of  Biological  Chemistry,  London 


TRANSLATED   FROM   THE  SECOND    GERMAN  EDITION  BY 

M.    M.    PATTISON    MUIR,    M.A. 

Fello-w  of  Canaille  and  Cains  College,  Cambridge 


WITH    58   ILLUSTBATIONS    BY    THE    AUTHOR 


NEW    YORK 

D.    VAN    NOSTRAND    COMPANY 

1901 


PRINTED  BY 

HAZELL,   WATSON,   AND  VINEY,    LD., 

LONDON  AND  AYLESBURY, 

ENGLAND. 


AUTHOR'S    PREFACE. 


IN  this  INTRODUCTION  TO  MODERN  SCIENTIFIC  CHEMISTRY 
an  attempt  is  made  to  give  a  succinct  and  accurate 
presentation  of  chemistry  on  strictly  scientific  lines, 
and  at  the  same  time  in  as  popular  a  form  as  is 
compatible  with  the  matter  and  the  vast  range  of 
the  subject.  The  book  can  be  followed  easily  by 
anyone  who  takes  a  serious  interest  in  natural 
science,  and  will  not,  I  hope,  be  unwelcome  to  the 
younger  chemists  who  are  still  pursuing  their  studies. 
A  teacher  of  chemistry  who  may  not  have  paid  special 
attention  to  the  methods  of  presenting  his  subject 
will  perhaps  find  in  the  book  something  useful  to  him- 
self and  helpful  to  his  hearers. 

The  form  followed  in  lectures  is  used  as  far  as 
possible ;  for,  by  pursuing  the  subject  in  this  way, 
the  reader  seems  to  find  for  himself  the  results  that 
chemistry  has  gained ;  and  experience  has  shown  that 
this  form  is  the  most  interesting  and  the  most  easily 
understood. 

V 

1 61895 


TABLE    OF    CONTENTS. 


PAGE 

INTRODUCTION k    .        l 

LIST  OF  THE  ELEMENTS 22 

HYDROGEN  GAS 28 

CHLORINE,  BROMINE,  IODINE,  FLUORINE,  AND  THEIR 

COMPOUNDS  WITH  HYDROGEN 40 

BROMINE .  .  .51 

IODINE 61 

FLUORINE .64 

THE  HYDROGEN  COMPOUNDS  OF  CHLORINE,  BROMINE, 

IODINE,  AND  FLUORINE 66 

HYDROCHLORIC  ACID  GAS  .  .  .  .  .  .  .66 

ACIDS,  BASES,  AND  SALTS 72 

HYDROBROMIC,  HYDRIODIC,  AND  HYDROFLUORIC  ACID     ..      76 
ATOMS  AND  THEIR  WEIGHTS     ......      78 

CALCULATING  FORMULAE  FROM  THE  RESULTS  OF 

ANALYSES •  .  .  .94 

MOLECULES  AND  THEIR  WEIGHTS  .  .  .  .  .99 

OXYGEN 113 

SULPHUR ,  142 

SULPHURIC  ACID 152 

ACID  SALTS.  DOUBLE  SALTS.  BASIC  SALTS  .  .  .158 
NITROGEN ,  ...  163 

vii 


Vlil  CONTENTS. 

PAGE 

NITRIC  ACID        .        .        . 172 

AQUA  REGIA 177 

EXPLOSIVES 177 

PHOSPHORUS ..'.'.        .  189 

THE  VARIOUS  MODIFICATIONS  OF  CERTAIN  ELEMENTS      .  194 

OZONE 197 

PHOSPHORETTED  HYDROGEN       .        .        .        .        .        .197 

THE     BUILDING     UP     OF     PLANTS     FROM     INORGANIC 

SUBSTANCES  .        .        .        ».-.,.        •.        .        .  204 

ARSENIC       .        .       .,-...-      .        .        .        .        .  217 

ANTIMONY    .        .        ...        .        .                 .        .  220 

CARBON        ....        .        .        .        .        .        .        .  222 

ORGANIC  CHEMISTRY  .        .        .     '  .        .        .        .        .  227 

THE  VALENCIES  OF  THE  ELEMENTS 227 

THE  CHEMISTRY  OF  ORGANISED  SUBSTANCES   .        .        .258 

THE  ASYMMETRIC  CARBON  ATOM 259 

THE  MANUFACTURE  OF  COAL-GAS 269 

ACETYLENE  GAS 274 

PETROLEUM  . 278 

FLAME  . 278 

SILICON 284 

THE  METALS 287 

THE  LIGHT  METALS 298 

PREPARATION  OF  THE  LIGHT  METALS  BY  ELECTRICITY    .  301 

POTASSIUM 303 

SODIUM 309 

CALCIUM 316 

MAGNESIUM 321 

ALUMINIUM 323 

THE  SYSTEMATIC  ARRANGEMENT  OF  THE  ELEMENTS         .  329 

INDEX 34* 


\  >>  •* 

V       Or   THE 

UNIVERSITY 

OF 


AN    INTRODUCTION 


TO 


MODERN  SCIENTIFIC   CHEMISTRY. 


BEFORE  entering  on  the  serious  study  of  Chemistry 
it  is  necessary  to  have  some  clear  notion  concerning 
the  range  and  scope  of  this  department  of  natural 
science.  For  the  name  Chemistry,  unlike  the  names 
of  many  parts  of  scientific  study,  does  not  tell  what 
is  the  sphere  of  the  science.  Everyone  knows,  for 
example,  that  Botany  is  concerned  with  plants,  Miner- 
alogy with  rocks,  Zoology  with  animals.  No  one  will 
confuse  Chemistry  with  these  departments  of  science, 
inasmuch  as  the  field  of  activity  of  each  is  circum- 
scribed by  its  name.  But  such  confusion  often  arises 
with  Physics,  because  the  circumstances  here  are  just 
the  same  as  with  Chemistry.  No  details  regarding  the 
questions  with  the  solution  whereof  Physics  is  con- 
cerned are  to  be  learnt  from  the  name  only.  It  is  not 
difficult,  however,  to  establish  the  difference  between 
the  problems  of  these  two  parts  of  natural  science, 
which  are  often  confounded.  In  doing  this  we  shall 
also  get  to  know  the  scope  and  range  of  Chemistry. 


2          INTRODUCTION    TO    MODERN    CHEMISTRY. 

Briefly  stated,  the  relations  between  these  two 
sciences  are  as  follows.  Chemistry  is  concerned  with 
the  investigation  of  processes  wherein  substances  are 
changed  into  other  substances  ;  Physics,  on  the  other 
hand,  examines  processes  wherein  no  changes  occur 
in  the  substances  which  exhibit  the  phenomena  that 
are  studied. 

Let  us,  for  example,  seize  a  bell  and  ring  it 
(figs.  I  and  2  are  given  as  helps  to  the 
memory  only).  We  all  hear  the 
sound ;  but  however  long  the  bell 
is  rung,  the  substance  of  it  re- 
mains unchanged.  Hence  the 
study  of  sound  belongs  to  Physics ; 
and  the  questions  to  which  that 
study  has  to  find  answers  are 
such  as  these  : — How  is  sound  generally  produced  ? 
In  what  way,  and  with  what  velocity,  is  it  propagated  ? 
Now  let  us  take  a  magnet  and  bring  it  near  a 
piece  of  iron  (fig.  2).  When  the  magnet  exerts  its 
mysterious  attractive  force,  which  looks  almost  like 
the  purpose  of  a  living  thing,  and  seizes  the  iron,  the 
magnet  itself  remains  unchanged.  If  we  remove 
the  piece  of  iron  that  has  adhered  to  the  magnet, 
the  latter  is  found  to  be  just  the  same  as  before, 
and  its  substance  to  be  unaltered.  Hence  the  in- 
vestigation of  magnetic  phenomena  also  is  a  problem 
of  Physics. 

But  if  we  now  set  fire  to  a  piece  of  paper  held  in 
small  pincers,  we  have  to  deal  with  a  very  different 
kind  of  phenomenon.  The  paper  burns,  and  at  the 
same  time  it  seems  to  vanish  altogether.  We  must, 


CHEMISTRY  AND   PHYSICS.  3 

of  course,  say  seems,  for  everyone  nowadays  has 
received  so  much  schooling  that  he  knows  that  matter 
cannot  be  destroyed,  but  can  only  be  changed  into 
other  forms.  And  so  what  was  a  piece  of  paper  now 
hovers  around  us  in  the  form  of  various  kinds  of 
gases,  which  were  produced  from  the  paper  while  it 
was  burning,  and  are  now  mixed  with  the  air  of  the 
room  wherein  we  are.  We  cannot  at  present  com- 
prehend what  kinds  of  gases  are  produced,  because 
we  have  not  yet  any  chemical  knowledge ;  but  this 
ignorance  will  not  last  very  long,  and  we  shall  be  able 
clearly  to  understand  what  exactly  has  been  produced 
from  this  piece  of  paper.  Nevertheless,  this  much  is 
certain — that  the  paper  must  now  be  something  quite 
different  from  what  it  was  before  the  burning,  and 
that  its  substance  has  been  wholly  altered.  In  pro- 
cesses of  burning,  then,  we  have  to  do  with  changes 
of  substances  ;  and,  consequently,  the  explanation  of 
combustion-processes  belongs  to  the  province  of 
Chemistry. 

The  marshalling  of  occurrences  as  belonging  to 
Chemistry  or  to  Physics  is  not  always  so  easy  as 
in  the  cases  that  have  been  chosen.  For  example, 
when  water  is  frozen  it  is  certainly  different  from 
liquid  water ;  and  water  that  is  evaporated  and  made 
to  take  the  form  of  water-vapour  is  also  different  from 
liquid  water.  But  there  can  be  no  talk  here  of  a 
change  of  the  substance  of  water,  like  the  change 
that  occurred  when  a  piece  of  paper  was  converted 
into  gaseous  substances.  We  can  easily  bring  any 
water  back  into  the  state  wherein  it  is  most  familiar  to 


4          INTRODUCTION    TO    MODERN    CHEMISTRY. 

us — the  liquid  state — either  by  warming  ice  or  cooling 
water-vapour.  Taking  into  consideration  such  changes 
of  substances  as  those  of  water,  we  must  slightly 
widen  the  import  of  the  definition  that  has  been  given 
of  the  business  of  chemistry.  The  definition  must 
run  thus  : — Chemistry  is  concerned  with  the  investigation 
of  processes  wherewith  changes  in  the  forms  of  sub- 
stances, and  also  essential  changes  in  the  composition 
of  substances,  are  connected. 

We  spoke  a  moment  ago  of  changing  water-vapour 
into  water  by  cooling  it.  Such  a  process  is  very  often 
carried  out  by  chemists,  not  only  with  water-vapour, 
but  also  with  vapours  of  many  kinds.  They  inten- 
tionally change  liquids  into  vapours,  and  by  cooling  the 
vapours  again  they  obtain  "  the  distillates  "  of  the  liquids. 
As  we  shall  be  using  distilled  water  very  often,  for  a 
reason  that  will  be  apparent  immediately,  it  is  advis- 
able, at  this  stage,  to  perform  a  process  of  distillation, 
and  especially  the  distillation  of  water.  For  this  pur- 
pose we  shall  use  (as  we  shall  always  use)  the  simplest 
possible  apparatus,  lest  complications  in  the  apparatus 
should  put  difficulties  in  the  way  of  recognising  what 
the  experiment  is  intended  to  demonstrate  and  display. 
In  order  to  distil  water,  we  boil  it  in  a  glass  flask.  It 
is  customary  not  to  put  such  a  glass  vessel  directly 
over  a  flame,  but  to  place  it  on  brass  or  iron  wire- 
gauze,  which  helps  to  spread  the  heat  over  the  bottom 
of  the  vessel,  and  so  prevents  the  breaking  of  it 
(see  fig.  3). 

The  glass  vessels  wherein  liquids  are  boiled  in  chemical  labo- 
ratories are  exceedingly  thin-walled — not  much  thicker  than  paper. 


DISTILLATION   OF  WATER.  5 

Consequently,  when  these  vessels  are  heated,  the  heat  spreads 
itself  throughout  their  material  almost  instantly ;  their  walls,  being 
equally  heated,  expand  equally,  and  cracking  is  avoided.  If  thick 
glass  is  heated,  strains  are  soon  developed  which  lead  to  the 
breaking  to  pieces  of  the  glass,  because  glass  is  a  very  bad 
conductor  of  heat.  If  a  glass 
drinking  -  vessel  is  placed 
over  a  flame,  the  outer  sur- 
face is  quickly  heated  and 
expands,  while  the  interior 
parts  remain  cold,  because 
of  the  small  thermal  con- 
ductivity of  the  glass,  and 
do  not  expand  at  all.  The 
great  strains  that  are  thus 
set  up  in  the  walls  of  such 
a  thick  vessel  cause  the 
glass  to  crack  and  break  to 
pieces. 

As  soon  as  the  water 
boils  we  see  what  we 
take  to  be  water- vapour 
issuing  from  the  mouth 
of  the  flask  (see  fig.  3). 
What  we  see  is  not 
really  water- vapour,  for 
the  vapour  of  water  is 
colourless;  and  of  this 
we  may  convince  our- 

,  ,  ,  ,  Fig.  3. — Boiling  water ;  water-vapour. 

selves  by  observing  that 

no  vapour  is  visible  inside  the  flask,  and  that  only  at 
the  mouth  of  the  flask  does  a  cloud  begin  to  be  seen. 
There  the  vapour,  which  is  at  the  temperature  of  100°  C. 
[212°  F.] — for  water  does  not  boil  until  that  temperature 
is  reached — comes  into  contact  with  the  air  of  the  room, 


6          INTRODUCTION    TO   MODERN    CHEMISTRY. 

which  is  colder  than  the  vapour;  and,  in  consequence 
of  the  sudden  cooling,  the  vapour  is  condensed  to 
exceedingly  fine  droplets  of  water,  which  we  see  floating 
in  the  air  and  we  call  water-vapour. 

If  we  place  a  cork,  through  a  hole  wherein  is  passed 
a  long  glass   tube  (bent  as    shown  in  fig.  4),  in  the 


Fig.  4.— Distillation  of  water. 

mouth  of  the  flask,  the  vapour  coming  from  the  boiling 
water  must  pass  through  this  long  tube.  The  water- 
vapour  will  then  be  much  cooled,  for  the  tube  will 
communicate  to  the  comparatively  cold  air  wherewith 
it  is  surrounded  the  heat  which  the  vapour  brings 
with  it.  The  result  of  this  cooling  process  is  that  the 
water-vapour  becomes  liquid  water.  We  see  this  liquid 


DISTILLATION   OF   WATER.  7 

water  dropping  into  the  vessel  A  (fig.  4),  placed  under 
the  outer  end  of  the  glass  tube.  The  water  which 
collects  in  the  vessel  A  is,  then,  distilled  water. 

The  apparatus  that  has  been  used  for  the  distillation 
of  water  is  the  most  simple  that  can  be  thought  of  for 


Fig.  5. —Distillation-apparatus  as  generally  used  in  laboratories. 

this  purpose.  That  apparatus  suffices  for  quite  a  few 
purposes  only;  for  the  glass  tube  which  acts  as  a  cooler 
soon  becomes  so  hot  that  a  large  portion  of  the  vapour 
passes  through  it  without  being  liquefied,  and  so  simply 
escapes  into  the  air,  and  is  lost  for  our  purposes.  A 
more  effective  cooling  apparatus  is  therefore  employed 
in  laboratories.  In  this  apparatus  (see  fig.  5)  the  tube  A, 


8          INTRODUCTION    TO   MODERN    CHEMISTRY. 

through  which  pass  the  vapours  to  be  cooled,  is  sur- 
rounded by  a  wider  tube  (B),  which  is  connected  with  A 
by  the  corks  c.  The  side-piece  D  (of  the  tube  B)  is 
connected  by  caoutchouc  tubing  with  the  water  supply : 
water  flows  between  the  tubes  B  and  A,  entering  at  D 
and  flowing  away  by  the  side-piece  E.  The  greater 
part  of  the  cooling-tube  A  is  thus  surrounded  by  cold 
water,  and,  being  kept  cold,  that  tube  is  able  to  condense 
to  liquid  all  the  vapour  that  comes  from  the  boiling- 
flask  H.  This  flask  (H)  carries  a  thermometer,  on  which 
is  read  off  the  temperature  of  the  vapour  that  passes 
over ;  for,  as  has  been  said  already,  liquids  of  diverse 
kinds  and  with  very  different  boiling  points  are  dis- 
tilled in  the  laboratory.  It  is  customary  not  to  allow 
the  distillate  to  drop  into  an  open  vessel,  as  was  done 
in  the  simpler  apparatus  shown  in  fig.  4,  but  to  collect 
it  in  a  flask  (K  in  the  figure),  into  which  the  end  of 
the  cooling-tube  passes. 

The  difference  between  the  water  which  we  have 
subjected  to  the  process  of  distillation  and  the  distillate 
obtained  therefrom  (that  is,  the  distilled  water)  is  as 
follows.  All  kinds  of  water  in  common  use  have  been 
in  contact  with  the  soil ;  and  every  soil  contains  sub- 
stances which  are  more  or  less  soluble  in  water.  If 
we  recollect  how  easily  sugar  dissolves  in  water,  we 
shall  realise  how  many  substances  may  be  kept  in 
solution  in  a  natural  water.  All  ordinary  waters  hold 
in  solution  very  diverse  substances  which  they  have 
taken  up  from  the  soil — for  example,  small  quan- 
tities of  carbonate  of  lime,  gypsum,  and  common  salt, 
three  substances  whose  names  do  not  sound  very 


DISTILLED  WATER.  9 

unfamiliar,  and  which,  for  that  reason,  will  be  the  only 
substances  we  shall  mention  at  present. 

Such  dissolved  substances  as  those  just  named  are 
not  volatile,  and  therefore  they  do  not  pass  over  with 
the  vapour  of  water  in  the  process  of  distillation,  but 
remain  behind  in  the  flask.  The  vapour  in  the  cooling- 
tube  is  free  from  these  substances,  and  therefore  the 
water  that  is  formed  by  cooling  this  vapour,  and  flows 
from  the  cooling-tube,  is  free  from  those  substances 
which  natural  waters 
take  up  from  the 
soils  :  it  is  pure 
water. 

In  order  to  make 
visible  the  difference 
between  natural  and 
distilled  water,  let  us 
put  some  water  that 

has  nnt  been  distilled    FiS-  6.— Behaviour  of  distilled  and  undistilled 

water  with  silver  nitrate  solution. 

into     the     glass     A 

(fig.  6),  and  some  distilled  water  into  the  glass  B,  and 
let  us  add  to  each  a  few  drops  of  a  solution  in  water 
of  nitrate  of  silver.  Silver  nitrate  is  a  salt  which  is 
very  soluble  in  water.  We  shall  learn  what  this  salt 
really  is  when  we  come  to  the  consideration  of  nitric 
acid  ;  *  at  present  a  solution  of  it  is  used  by  us  only 
as  a  means  to  an  end.  We  notice  that  the  distilled 
water  in  B  is  not  changed  by  the  addition  of  the  silver 

*  References  to  later  parts  of  the  book  should  be  located  by 
the  index. 


10       INTRODUCTION   TO   MODERN   CHEMISTRY. 

nitrate  solution,  but  that  the  natural  water  in  A  is 
immediately  made  turbid  and  appears  like  milk.  The 
reason  for  this  is  as  follows.  The  distilled  water, 
which  is  nothing  but  water,  only  dilutes  the  solution 
of  silver  nitrate — that  is,  only  adds  more  water  to  what 
is  ajready  there  ;  the  ingredients  of  the  ordinary  water, 
on  the  other  hand,  react  with  the  nitrate  of  silver  that 
is  present  in  the  solution  which  was  added.  Carbonate 
of  silver,  which  is  not  soluble  in  water,  and  other 
insoluble  compounds  of  silver,  have  been  formed  in 
the  water  in  A,  and  wre  see  these  solid  substances 
floating  in  the  water  and  causing  it  to  be  turbid  and 
like  milk.  Ordinary  water  cannot  be  used  as  a  solvent 
or  a  diluent  in  accurate  chemical  experiments,  be- 
cause the  substances  that  are  dissolved  in  ordinary 
waters  act  on  solutions  of  various  bodies  in  different 
ways — we  have  just  had  a  special  example  of  such 
actions — and  the  substances  that  are  produced  inter- 
fere with  the  reactions  which  it  is  desired  to  study. 
Water  must  be  distilled  before  it  is  used  in  the  labo- 
ratory, in  order  to  remove  those  substances  which,  if 
present,  would  exert  disturbing  actions.  Thus  we  see 
why  chemists  almost  always  use  distilled  water. 

Now  that  we  know  what  distillation  is  and  what 
purpose  it  serves,  we  shall  return  to  chemical  experi- 
ments ;  and  we  shall  begin  by  allowing  sulphur  to 
react  with  iron.  Sulphur  has  nothing  mysterious  about 
it,  although  many  people  may  think  it  is  a  strange  sub- 
stance, because  they  are  not  accustomed  to  have  to  do 
with  it.  Quantities  of  sulphur  are  found  in  the  soil, 
for  instance,  in  Sicily  :  it  is  only  necessary  to  separate 


COMBINING    IRON    WITH   SULPHUR. 


II 


the  sulphur  from  the  rocks  wherewith  it  may  be  mixed 
in  order  to  fit  it  for  use.  But  it  is  otherwise  with  iron. 
Except  in  some  meteorites,  iron  itself  is  not  found  in 
the  earth;  it  must  be  produced,  by  very  complicated 
processes,  which  we  shall  become  acquainted  with  later, 
from  the  compounds  of  it  which  are  found  in  the  earth's 
crust.  Although  iron  is  much  more  difficult  to  prepare 
than  sulphur,  yet,  consider- 
ing that  it  is  a  substance  in 
daily  use,  it  does  not  seem 
to  be  especially  worth  a 
close  examination.  At  pre- 
sent we  require  both  sulphur 
and  iron  as  finely  powdered 
as  we  can  get  them. 

We  now  mix  the  pow- 
dered iron  with  the  pow- 
dered sulphur.  They  do 
not  react  on  one  another 
without  further  treatment. 
If,  for  instance,  we  move  =E= 
our  magnet  in  the  mixture, 
the  iron  is  drawn  out  by  the 

,     .         .»•  Fig.  7.— Preparation  of  iron  sulphide. 

magnet;    and  in    this    way 

the  two  substances  can  be  separated  from  one  another. 
Let  us  now  pour  the  mixture  into  a  test-tube.  A 
test-tube  is  a  tube  of  glass  closed  at  one  end  by 
melting  the  glass  and  letting  it  flow  together;  these 
tubes  are  very  much  used  in  making  experiments.  We 
place  the  test-tube  A,  half  filled  with  the  mixture,  in 
a  clamp,  and  heat  it  by  a  flame  (see  fig.  7).  We  see 
the  contents  of  the  tube  suddenly  begin  to  glow  at  one 


12       INTRODUCTION   TO   MODERN    CHEMISTRY. 

point ;  and  although  we  at  once  remove  the  flame  and 
thereby  cease  to  add  heat,  the  glowing  continues  and 
spreads  throughout  the  whole  mass. 

When  the  mass  has  become  cold  and  we  examine  it 
to  see  what  has  happened,  instead  of  the  yellow  sulphur 
and  the  grey  iron,  we  find  a  brownish  black  body, 
which  has  been  melted,  and  is  neither  the  one  nor  the 
other  of  the  two  materials  wherewith  we  started.  The 
magnet  has  no  effect  on  the  new  substance,  and  the 
most  powerful  microscope  fails  to  reveal  particles  either 
of  iron  or  sulphur  in  it.  We  have  to  do  here  with  an 
occurrence  that  belongs  to  the  domain  of  chemistry. 
A  compound  of  the  two  bodies  has  been  produced,  and 
this  compound  exhibits  neither  the  properties  of  sulphur 
nor  those  of  iron  :  a  transformation  has  been  effected  of 
the  two  substances  into  a  third — new — substance. 

Experiments  conducted  like  that  just  described, 
whereby  we  gain  new  bodies,  are  called  synthetical 
experiments.  We  have  carried  out  the  synthesis  of 
sulphide  of  iron.  But,  of  course,  we  can  reverse 
our  method  of  working.  We  can  try  to  answer  the 
questions  : — Of  what  sabstances  are  the  various  bodies 
formed  which  are  around  us  ?  Are  we  in  a  position 
to  separate  them  into  simpler  substances  ?  This 
method  of  working,  which  is  opposed  to  the  syn- 
thetical method,  is  called  analysis. 

Now  that  we  have  become  acquainted  with  a  syn- 
thesis, we  shall  perform  an  analysis.  That  which  is 
nearest  our  hand  is  to  decompose  the  sulphide  of  iron 
we  have  made  into  its  constituents — to  obtain  from  it 
sulphur  and  iron.  But  that  analysis  would  be  too 


ANALYSIS   OF    MERCURY   OXIDE.  13 

difficult  to  make  clear  at  this  stage  of  our  progress, 
when  we  have  hardly  any  chemical  knowledge. 

Just  as  we  used  iron  and  sulphur  for  conducting  a 
synthesis,  because  these  substances  seemed  especially 
suitable  for  our  method  of  introducing  the  reader  to  a 
knowledge  of  chemistry,  so  we  must  select  some  sub- 
stance especially  suited  for  carrying  out  an  analytical 
process. 

Oxide  of  mercury  will  serve  us  well  for  this  purpose. 
It  is  a  red  powder,  found  in  commerce :  we  shall  learn 
how  to  prepare  it  at  a  later  time.  The  word  oxide t  which 
is  so  often  heard,  is  derived  from  the  name  oxygen.  As 
sulphide  of  iron  is  composed  of  sulphur  and  iron,  so  oxide 
of  mercury  (or  mercury  oxide)  is  a  compound  of  oxygen 
and  mercury.  If  we  carry  out  the  analysis  of  mercury 
oxide,  the  substance  will  thereby  be  decomposed  into 
mercury  and  oxygen.  The  solution  of  this  analytical 
problem  is  not  difficult,  because  heating  the  compound 
causes  its  decomposition ;  hence,  if  we  bring  our  red 
powder  to  a  sufficiently  high  temperature,  in  a  properly 
arranged  apparatus,  we  shallobtain  mercury  and  oxygen 
gas.  In  this  experiment  we  obtain  a  gas,  besides  a 
metal ;  hence,  before  conducting  the  analysis  we  must 
learn  something  about  general  methods  of  manipulating 
and  working  with  gases.  To  do  that  is  not  particularly 
difficult — at  least,  not  so  far  as  we  need  go  into  the 
matter.  But  as  gaseous  bodies  must  play  an  important 
part  in  an  introduction  to  chemistry,  we  shall  examine 
this  subject  somewhat  in  detail. 

For  working  with  gases  a  basin  containing  water  is 
required ;  this  basin,  which  may  be  round  or  with 


14       INTRODUCTION    TO   MODERN   CHEMISTRY. 

corners,  is  called  a  pneumatic  trough  (A,  figs.  8  and  9). 
A  vessel  is  needed  for  collecting  the  gas  to  be  ex- 
amined ;  this  vessel  is  made  of  glass,  and  is  generally 
cylindrical  in  form.  A  strip  of  metal,  pierced  with 
holes,  and  called  a  shelf,  hangs  in  the  water  in  the 


Fig.  8.— Apparatus  for  working  with  gases  :  pneumatic  trough,  shelf,  and 
cylinder ;  also  method  of  filling  a  cylinder  with  water. 

trough  (B,  figs  8  and  9).  This  is  all  the  apparatus 
required.  If  a  glass  cylinder  with  a  foot  is  laid  in  the 
water  in  the  trough  so  that  it  is  entirely  filled  with 
water  (see  fig.  8),  and  it  is  then  raised  (with  the  foot 
upwards)  without  removing  the  open  end  from  under 
the  water  (see  fig.  9),  the  water  in  the  cylinder  will, 


COLLECTING   GASES. 


5 


of  course,  be  prevented  from  flowing  out  by  the  pressure 
of  the  air  outside  the  cylinder.  We  are  taught  by 
physics  that  the  atmospheric  pressure  will  balance  the 
pressure  of  a  column  of  water  about  ten  metres  in 
height;  our  cylinder  might,  therefore,  be  ten  metres 
long  without  there  being  any  fear  of  the  water  flowing 
out  when  it  was  set  up  on  end  in  the  manner  just 
described.  When  the  cylin- 
der c  has  been  filled  with 
water,  and  set  upright  with 
its  open  end  under  the 
water,  it  is  pushed  on  to 
the  shelf  B,  of  course  with- 
out removing  it  from  the 
water  in  the  trough.  We 
have  now  what  we  wanted 
— namely,  a  space  wherein 
we  can  collect  a  gas.  If 
we  allow  a  gas  to  enter 
the  cylinder  through  one 
of  the  holes  in  the  shelf 
(placed  under  the  cylinder), 
as  is  shown  in  fig.  10,  the  light  gas  will  rise  through 
the  water  in  the  form  of  bubbles,  and  will  collect 
above  the  water,  which  it  will  drive  out  of  the  cylinder. 

If  a  gas  must  not  be  collected  over  water,  some  other  liquid — 
mercury,  for  instance— may  be  used  in  the  place  of  water.  The 
method  of  collecting  gases  over  mercury  differs  in  no  respects 
from  that  which  we  have  been  using  with  water  as  the  liquid. 
Fig.  43  represents  an  example  of  the  use  of  mercury. 

The  analysis  of  mercury  oxide  is  carried  out  in  the 
following  way.  The  red  powder  is  shaken  into  the 


Fig.  9. — Cylinder  filled  with  water, 
and  inverted  in  water  in  pneu- 
matic trough. 


1 6        INTRODUCTION    TO   MODERN    CHEMISTRY. 

retort  A  (fig.  10).  The  receiver  v  is  joined,  air-tight,  to 
the  retort.  The  suitably  bent  glass  tube  E  passes 
through  a  cork,  which  fits  tightly  into  the  opening  D  of 
the  receiver  v ;  the  lower  end  of  the  glass  tube  passes 
under  the  water  in  the  trough ;  this  tube  will  serve  to 
lead  the  oxygen  gas  under  the  shelf  B,  so  that  it  may 


Fig.  io.— Decomposition  of  oxide  of  mercury  into  mercury  and  oxygen,  and 
collection  of  the  oxygen. 

reach  the  cylinder  c  and  collect  therein,  At  the  be- 
ginning of  the  experiment  the  open  end  of  the  gas- 
conducting  tube  should  be  under  the  water,  but  not  yet 
under  the  shelf.  If  we  now  heat  the  mercury  oxide  in 
the  retort,  by  placing  a  lamp  beneath  it,  we  at  once  see 
gas-bubbles  escaping  from  the  open  end  of  the  gas- 
conducting  tube,  rising  in  the  water  of  the  trough  and 
issuing  into  the  air.  The  bubbles  which  appear  at  the 


COLLECTING  AND  EXAMINING  OXYGEN    17 

beginning  of  the  experiment  are  nothing  but  air,  which 
is  expanded  by  the  heat  in  the  retort,  and,  not  finding 
room  enough  in  the  apparatus,  issues  from  the  end  of 
the  glass  tube  that  opens  under  the  water.  Soon, 
however,  gas-bubbles  begin  to  rise  more  regularly  and 
in  greater  abundance.  These,  which  partly  consist  of 
oxygen,  are  allowed  to  escape  for  a  time,  in  order  that 
they  may  carry  with  them  the  air  from  the  retort  and 
the  other  parts  of  the  apparatus.  After  about  two 
minutes  this  has  been  accomplished ;  and  now,  as  pure 
oxygen  is  coming  off,  we  move  the  end  of  the  gas- 
conducting  tube  under  the  shelf,  so  that  the  gas  may 
rise  into  the  cylinder  and  collect  therein,  as  shown  in 
fig.  10. 

In  order  to  prove  that  the  gas  which  has  been 
gradually  filling  the  cylinder  is  oxygen,  we  make  use  of 
the  property  of  this  gas  to  maintain  combustion  in  the 
most  vigorous  way,  and  to  act  on  burning  or  glowing 
bodies  much  more  energetically  than  air,  so  that  a  slip 
of  wood  which  just  glows,  and  is  therefore  about  to 
expire,  in  the  air,  bursts  into  flame  immediately  it  is 
brought  into  this  gas.  For  this  purpose  we  slip  the 
cylinder  off  the  shelf  into  the  open  water,  and  place  a 
glass  plate  on  its  open  end  under  the  water.  Then  we 
raise  the  cylinder  from  the  trough,  reverse  it,  and  place 
it  on  the  table  in  its  ordinary  position,  with  the  open 
end  upwards.  At  the  moment  when  we  remove  the 
glass  plate  which  has  been  closing  the  mouth  of  the 
cylinder  'we  plunge  a  glowing  slip  of  wood  into 
the  vessel.  At  once  we  see  the  slip  of  wood  burn- 
ing brightly  within  the  cylinder  (that  is,  in  the  gas 

2 


1 8       INTRODUCTION    TO    MODERN    CHEMISTRY. 

contained  therein),    whereby   the   presence   of  oxygen 
gas  is  demonstrated. 

It  should  be  noted  that  the  end  of  the  gas-conducting  tube  which 
is  in  the  trough  should  always  be  removed  from  the  water  before 
a  process  wherein  gas  is  produced  is  discontinued— before,  for 
instance,  the  flame  is  removed  from  under  the  vessel  wherein  the 
gas  is  produced. 

If  this  is  not  done,  water  will  be  driven  back  into  the  apparatus 
by  the  pressure  of  the  air,  because  the  gas  in  the  apparatus  will 
contract  as  it  cools.  If  the  apparatus  is  hot,  the  cold  water  driven 
into  it  will  probably  cause  it  to  break  ;  in  any  case,  the  water 
will  make  the  contents  of  the  apparatus  wet.  If  the  open  end  of 
the  glass  tube  is  in  the  air,  in  place  of  in  water,  air  will  enter  the 
apparatus :  but  this  does  no  harm,  for  when  the  apparatus  is 
thoroughly  cold  it  is  in  the  same  condition  as  it  was  before  the 
experiment  began ;  it  contains  air,  as  it  did  then. 

If  we  now  look  more  closely  at  the  receiver  v  con- 
nected with  the  retort,  we  notice  that  some  mercury  has 
collected  therein.  Mercury  is  volatilised — that  is,  it 
distils  just  as  we  saw  water  distil — at  the  high  tempera- 
ture needed  for  the  decomposition  of  mercury  oxide 
into  its  two  constituents.  The  vapour  of  the  mercury 
is  condensed  to  liquid  mercury  by  cooling  in  the 
receiver. 

Our  analysis  is  now  finished.  We  have  done  what 
we  proposed  to  do  ;  we  have  separated  mercury  oxide 
into  the  two  substances  whereof  it  is  composed — 
mercury  and  oxygen  gas.* 

*  Concerning  the  use  of  the  words  gas  and  vapour,  it  should 
be  noticed  that  it  is  customary  to  speak  of  gases  as  those  kinds 
of  airs  (including  common  air  itself)  which  are  not  changed  to 
liquids  at  the  ordinary  temperature,  and  of  vapours  as  those 
kinds  of  airs  which  become  liquids  at  the  ordinary  temperature. 


THE   ELEMENTS.  19 

The  most  appropriate  thing  to  do  now  would  cer- 
tainly be  to  endeavour  to  analyse,  more  completely  to 
decompose,  the  mercury  and  the  oxygen  which  we 
have  just  obtained.  Were  we  to  make  that  attempt,  we 
should  find  that  all  our  labour  was  in  vain.  Both  of 
the  substances  obtained  from  mercury  oxide  turn  out 
to  be  undecomposable.  Undecomposable  substances, 
such  as  the  two  we  have  just  recognised — substances 
that  resist  all  analysis  and  must  therefore  be  looked 
on  as  not  composed  of  more  than  one  thing — :are  called 
ELEMENTS  in  chemistry. 

Our  very  simple  analysis  has  brought  us  at  once  to 
the  elements.  During  the  hundreds  of  years  chemists 
have  been  examining  all  our  material  surroundings 
everything  has  been  analysed,  in  order  to  find  out  the 
elements — the  constituents  of  bodies  which  cannot 
themselves  be  further  decomposed,  and  by  whose 
union  these  bodies  are  formed.  Nothing  has  been 
forgotten,  whether  we  think  of  the  colouring  materials 
wherewith  the  ceilings  of  our  rooms  are  covered,  or  the 
bricks  wherewith  our  houses  are  built,  or  the  hair  of 
our  heads,  or  the  coal-gas  by  whose  burning  our  rooms 
are  lighted  in  the  evening.  And  these  analyses  of  the 
numberless  things  we  see  on  the  earth  have  brought 
to  light  the  remarkable  fact  that  nature  produces  all 
these  things  with  the  help  of  an  astonishingly  small 
number  of  elements.  About  seventy-five  elements 
are  knwon  to-day  :  this  number  cannot  be  said  to  be 
very  large. 

Now  it  also  happens  that  most  of  the  elements 
are  so  rare  that  they  need  not  be  taken  into  account 


20        INTRODUCTION    TO   MODERN    CHEMISTRY. 

by  those  who  are  not  chemists,  since  they  have 
no  influence  on  the  composition  of  the  common 
things  around  us.  To  take  an  example  :  the 
element  germanium,  discovered  in  1886,  was  ob- 
tained from  a  mineral,  unknown  before  that  time, 
which  was  found  in  a  very  deep  working  in  a  mine 
at  Freiberg,  in  Saxony.  This  mineral,  of  which 
very  small  quantities  are  found  in  the  workings  re- 
ferred to,  is  the  source  of  germanium,  although  traces 
of  that  element  have  also  been  found  in  certain  other 
very  rare  materials;  hence  we  can  understand  that 
to  obtain  germanium  in  considerable  quantities—- 
if,  indeed,  that  could  be  done  at  all — would  be  more 
difficult  than  to  get  quantities  of,  say,  diamonds, 
which  can  be  had  by  the  bushel  if  only  one  will 
pay  for  them. 

When,  by  their  analyses,  chemists  determine  the 
elements  whereof  the  substances  they  examine  are 
composed,  they  find  that,  speaking  broadly  and  leaving 
out  of  account  the  rarer  minerals,  about  twenty-five 
elements  enter  into  the  compositions  of  these  sub- 
stances. As  the  elements  found  in  this  or  that  sub- 
stance could  not  be  carried  in  the  memory,  it  was 
necessary  to  note  down  the  results  of  analyses — that 
is  to  say,  the  names  of  the  elements.  Of  course, 
chemists  would  not  write  these  few  names  in  full 
every  time,  but  from  constant  use  they  would  learn 
to  recognise  what  element  was  meant  when  only 
one  or  two  letters  of  the  name  were  noted.  The 
custom  of  chemists  of  designating  the  elements  by  the 
initial  letters  of  their  names  had  its  origin  somewhat 


ABBREVIATED   NAMES   OF   ELEMENTS.  21 

in  this  way.  A  similar  device  of  shortening  names  is 
used  in  many  cases  in  ordinary  affairs  ;  for  instance, 
there  is  the  custom  of  not  writing  the  names  of  well- 
known  people  in  full,  because  we  know  the  significa- 
tion of  the  contractions  in  common  use.  To  take  an 
example  :  everyone  who  read  in  Punch's  Essence  of 
Parliament  of  Mr.  G.  knew  that  contraction  to  refer 
to  the  Right  Honourable  W.  E.  Gladstone.*  For 
phosphorus  chemists  write  P,  for  bromine  Br,  for 
aluminium  Al ;  and  we  can  understand  these  con- 
tractions without  any  trouble  when  we  find  them  in 
books  which  deal  with  chemistry.  It  is  to  be  re- 
marked here  that,  as  learned  people  used  the  Latin 
language  in  the  old  days,  the  contracted  form  of 
the  names  of  many  elements  which  have  been 
long  known  are  derived  from  the  Latin  names  of 
these  elements ;  for  instance,  for  iron  one  writes 
Fe  (from  the  Latin  /ermm),  and  for  gold  Au  (from 
the  Latin  aurum}.  Every  chemist,  to  whatever 
country  he  may  belong,  uses  the  same  contrac- 
tions ;  hence  these  contracted  forms  are  equally  well 
understood  by  all.  It  is  scarcely  necessary  to  say 
that  no  sort  of  mystery  is  to  be  attached  to  these 
abbreviations. 

We  shall  now  give  an  alphabetical  list  of  the  names 
of  the  elements  at  present  known,  and  opposite  each 
name  we  shall  place  the  abbreviated  symbol  of  the 
element  that  is  employed  everywhere. 

*  The  example  given  in  the  text  is  Just.  v.  Liebig,  which  every 
German  knows  to  stand  for  Justus  von  Liebig. 


22        INTRODUCTION   TO   MODERN    CHEMISTRY. 


LIST  OF  THE  ELEMENTS, 

AND   THE   ABBREVIATED   METHOD   OF  WRITING   THEIR   NAMES. 


Name  of  Element. 

Shortened 
form  of 
Name. 

Name  of  Element. 

Shortened 
form  of 
Name. 

Aluminium      .         .          Al 

Molybdenum  . 

Mo 

Antimony  (stibium).         Sb 

Neodymium    . 

Nd 

Argon     ...         A 

Neon 

Ne 

Arsenic  .         .         .As 

Nickel     . 

Ni 

Barium   .         .         .  i       Ba 

Niobium  . 

Nb 

Beryllium        .         .         Be 

Nitrogen. 

N 

Bismuth  .         .         .  |       Bi 

Osmium. 

Os 

Boron                                B 

Oxygen  . 

0 

Bromine.         .         .         Br 

Palladium 

Pd 

Cadmium 

Cd 

Phosphorus     . 

P 

Caesium 

Cs 

Platinum 

Pt 

Calcium  . 

Ca 

Potassium  (kalium} 

K 

Carbon    . 

C 

Praseodymium 

Pr 

Cerium   . 

Ce 

Rhodium 

Rh 

Chlorine 

Cl 

Rubidium 

Rb 

Chromium 

Cr 

Ruthenium 

Ru 

Cobalt     . 

Co 

Samarium 

Sa 

Copper  (cuprum]    . 

Cu 

Scandium 

Sc 

Erbium  . 

Er 

Selenion 

Se 

Fluorine  . 

F 

Silicon    . 

Si 

Gallium  . 

Ga 

Silver  (argentum)  . 

Ag 

Germanium     . 

Ge 

Sodium  (natrium)  . 

Na 

Gold  (aurum). 

Au 

Strontium 

Sr 

Helium   .         .         * 

He 

Sulphur  . 

S 

Hydrogen 

H 

Tantalum 

Ta 

Indium    . 

In 

Tellurium 

Te 

Iodine     .         .         .  j       I 

Thallium 

Tl 

Iridium  .         .         .          Ir 

Thorium. 

Th 

Iron  (ferrum)         .         Fe 

Tin  (stannum) 

Sn 

Krypton                               Kr 

Titanium 

Ti 

Lanthanum     .         .La 

Tungsten  (wolfram} 

W 

Lead  {plumbum}    .         Pb 

Uranium 

U 

Lithium  .         .         .  |       Li 

Vanadium 

V 

Magnesium     .         .          Mg 

Xeon 

Xe 

Manganese      .         .  J       Mn 

Ytterbium 

Yb 

Mercury      (liydrar-  ' 

Yttrium  . 

Y 

gyrum}        .                  Hg 

Zinc 

Zn 

Metargon         .         .  ,       Mt 

Zirconium 

Zr 

CHEMICAL   FORMULA  AND   EQUATIONS.          23 

With  the  help  of  these  contracted  forms,  let  us  now 
write  the  two  chemical  compounds  wherewith  we  are 
fairly  familiar — namely,  iron  sulphide  and  mercury 
oxide.  The  process  is  very  simple.  The  table  shows 
that  the  abbreviation  for  sulphur  is  S,  and  for  iron  Fe, 
hence  the  letters  SFe  mean  iron  sulphide  to  the 
chemist ;  and  as  the  contractions  for  mercury  and 
oxygen  respectively  are  Hg  and  O,  he  writes  mercury 
oxide  HgO.  Such  shortened  forms  of  writing  are 
called  the  formulae  of  the  compounds ;  SFe  is  the 
chemical  formula  of  iron  sulphide,  and  HgO  is  the 
chemical  formula  of  mercury  oxide. 

To  go  a  step  farther.  Not  only  can  the  chemical 
compounds  be  indicated  by  using  the  abbreviated 
forms  of  the  names  of  the  elements,  but  the  chemical 
processes  also  whereby  these  compounds  are  formed 
can  be  expressed  in  a  very  simple  way.  Such  expres- 
sions are  called  chemical  equations. 

For  this  purpose  the  sign  +  is  placed  between  the 
formulae  of  the  substances  which  interact,  and  the 
formula  of  the  substance  formed  is  placed  after  the  sign 
of  equality.  The  formation  of  iron  sulphide  is  ex- 
pressed as  follows  : — 

Sulphur  and  iron  produce  iron  sulphide ;  or,  using 
the  abbreviations,  S  +  Fe  =  SFe. 

Such  chemical  equations  make  it  possible  to  express 
the  results  of  the  chemical  reactions  of  substances  in  a 
very  clear,  brief,  and  precise  way. 

However  pleasing  and  satisfactory  may  be  this  clear 
method  of  expressing  chemical  reactions  by  means  of 
the  shortened  designations  of  the  elements,  it  is  not  to  be 


24       INTRODUCTION    TO   MODERN   CHEMISTRY. 

denied  that,  so  far  as  our  experience  of  the  use  of  these 
contractions  has  gone,  one  might  doubt  whether  much 
is  to  be  gained  from  what  may  be  called  a  special  way 
of  writing  the  expression  of  thoughts.  To  write  the 
names  of  the  elements  in  full  would  serve  the  purpose 
in  view,  and  would  save  one  the  trouble  of  learning 
to  understand  this  special  method,  which,  it  must  be 
confessed,  savours  somewhat  of  stenography.  But  in 
a  later  part  of  this  book,  when  we  shall  have  more 
chemical  knowledge,  we  shall  learn  the  full  meanings 
of  these  shortened  expressions,  which  stand  for  much 
more  than  merely  the  names  of  the  elements,  and, 
understanding  them,  we  shall  be  able  to  estimate  their 
value.  We  shall  perceive  why  these  abbreviated  ex- 
pressions are  indispensable  to  chemists,  and  why, 
considering  the  matter  from  the  position  whereat 
chemistry  has  now  arrived,  it  is  impossible  even  to 
think  of  giving  them  up. 

If  we  now  turn  over  in  our  minds  what  will  be  the 
best  method  of  proceeding  in  order  to  advance  from 
the  desultory  information  we  have  acquired  to  connected 
chemical  knowledge,  we  shall  be  convinced  that  without 
doubt  the  most  suitable  method  would  be  to  bring 
one  element  after  another,  and  the  compounds  of  one 
element  after  those  of  another,  within  the  range  of 
our  consideration.  By  following  this  plan  we  should 
certainly  overlook  nothing ;  for,  as  all  things  are 
composed  of  the  small  number  of  elements  given  in 
the  table  on  p.  22,  this  method  would  involve  the 
consideration  of  all  material  substances  that  exist. 
The  complete  carrying  out  of  this  principle  would, 


SPECIFIC   GRAVITY.  25 

of  course,  demand  unlimited  time ;  nevertheless,  the 
idea  whereon  the  plan  is  based  is  certainly  sound. 
We  must  limit  ourselves  to  this  extent  only  :  we  must 
confine  our  attention  to  certain  selected  elements  and 
their  compounds.  By  reviewing  these  bodies  we  shall 
gain  what  we  are  chiefly  concerned  to  gain  in  this 
introduction  to  chemistry — some  knowledge  of  those 
chemical  occurrences  which  are  especially  important ; 
and  we  shall  be  able  to  draw  conclusions  of  general 
applicability  and  general  interest,  some  of  which  extend 
far  beyond  the  special  domain  of  chemistry. 
With  which  element,  then,  shall  we  begin  ? 

The  elements  may  be  solid,  liquid,  or  gaseous. 
Iron,  for  instance,  is  a  solid,  bromine  a  liquid,  and 
hydrogen  a  gas.  The  elements  are  divided  into  two 
main  classes — the  metals  and  the  non-metals.  What 
kind  of  elements  the  metals  are  follows  from  the  ideas 
we  connect  with  the  word  metal  in  ordinary  life.  The 
non-metals  are  those  elements  which  have  not  metallic 
properties.  We  shall  begin  with  an  element  belonging 
to  the  class  of  non-metals — namely,  hydrogen,  which 
is  the  specifically  lightest  body  that  is  known  to  us. 

At  this  point  it  may  be  allowed  to  elucidate  the 
notion  of  specific  gravity — a  notion  we  shall  frequently 
use,  and  one  which,  perhaps,  is  not  familiar  to  every 
reader.  Suppose  that  a  glass  flask  whose  capacity,  to 
a  mark  on  the  neck,  is  exactly  one  litre  (see  fig.  n)  is 
filled  with  hydrogen ;  then,  if  the  flask  is  weighed  on  a 
balance,  the  weight  of  it  is  less  than  would  be  the 
weight  of  the  same  flask  filled  with  any  other  substance. 


26       INTRODUCTION    TO   MODERN   CHEMISTRY. 


We  should  find  that  a  litre  of  hydrogen  weighs  only 
•0896  gram,  whereas  a  litre  of  air,  for  example,  weighs 
1-295  grams.  By  dividing  one  of  these  numbers  by  the 
other  we  see  that  hydrogen  is  14*4  times  lighter  than 
air.  The  density  of  hydrogen  is  one  I4'4th  part  of  that 
of  air.  We  should,  of  course,  have  arrived  at  the  same 
result  had  we  compared  the  weights  of  any  equal 
volumes  of  hydrogen  and  air  ;  it  is  not  necessary  that 
one  litre  of  each  should  be 
chosen.  Specific  gravity  ex- 
presses, in  a  completely  general 
way,  the  relation  between  the 
weights  of  those  quantities  of 
two  substances  which  occupy 
equal  volumes. 

While  the  specific  gravities 
of  gases  are  referred  to  hydro- 
gen as  the  unit — or,  as  is 
often  said,  to  hydrogen  taken 
as  unity — the  specific  gravities 
of  all  liquid  and  solid  bodies 
are  referred  to  water  as  unity, 
because  if  these  also  were  referred  to  hydrogen  the 
numbers  expressing  the  specific  gravities  of  solids  and 
liquids  would  be  inconveniently  large.  For  instance, 
the  specific  gravity  of  iron  is  7*78 ;  this  tells  us  that,  as 
one  cubic  metre  of  water  weighs  1000  kilograms,  one 
cubic  metre  of  iron  weighs  7780  kilograms.  Let  us 
calculate  how  much  lighter  hydrogen  is  than  water. 
A  litre  of  hydrogen  weighs  -0896  gram ;  a  litre  of 
water  weighs  1000  grams  :  by  dividing  1000  by  '0896 
we  obtain  the  desired  number — namely,  11,160. 


Fig.  IT. — Litre'.flask. 


SPECIFIC   GRAVITY.  27 

Hydrogen,  then,  is  1 1, 1 60  times  lighter  than  water; 
in  other  words,  11,160  litres  of  hydrogen  weigh  one 
kilogram. 

It  may  not  be  without  interest  to  learn  what  is  the  specifically 
heaviest  metal,  and  thus  to  know  what  is  the  specifically  heaviest 
substance  found  in  nature.  This  substance  is  the  metal  osmium. 
The  specific  gravity  of  this  metal  is  22-48,  referred  to  water  as 
unity  :  hence  a  piece  of  osmium  of  the  bulk  of  one  litre  weighs 
22,480  grams — that  is,  250,893  times  as  much  as  a  litre  of  hydro- 
gen ;  in  other  words,  the  specific  gravity  of  osmium  is  250,893 
referred  to  hydrogen  as  unity. 


HYDROGEN    GAS. 

THE  name  hydrogen  [=  water  producer]  indicates  that 
this  gas  is  an  essential  constituent  of  water.  The 
chemical  analysis  of  water  shows  that  substance  to 
consist  of  two  gases.  Because  it  occurs  in  water,  one 
of  these  gases  is  called  hydrogen;  the  other  gas, 
which,  by  uniting  with  hydrogen,  forms  water,  was 
obtained  by  us  by  analysing  mercury  oxide — it  is 
oxygen. 

After  what  we  have  already  seen,  we  need  not  be 
at  all  astonished  that  two  gases  should  unite  to  form 
a  liquid.  Mercury  oxide,  which  we  analysed,  is  a 
solid  red  body ;  when  we  decomposed  it  into  its 
elements  we  found  that  it  consists  of  mercury,  which 
is  a  liquid,  and  oxygen,  which  is  a  gas.  In  that  case 
we  had  to  do  with  a  solid  body,  which  proved  to  be 
composed  of  a  liquid  and  a  gas ;  now  we  have  to  do 
with  the  liquid  body  water,  which  is  composed  of  two 
gases.  In  short — and  of  this  we  shall  have  many 
examples  as  we  proceed — the  purely  external  char- 
acters of  the  elements  are  of  no  importance  when  we 
are  considering  the  compounds  which  these  elements 
form  by  uniting  with  one  another. 


PREPARATION  OF  HYDROGEN.        29 

If  we  could  find  anything  for  which  oxygen  has  a 
greater  affinity  than  it  has  for  hydrogen — anything 
wherewith  oxygen  would  unite  in  preference  to  uniting 
with  hydrogen — we  should  be  able  to  separate  the  two 
elements  that  are  united  in  water.  Oxygen  will  unite 
with  any  body  by  which  it  is  more  attracted  than  by 
hydrogen  :  when  this  occurs,  the  hydrogen  of  the  water 
will  be  set  free ;  and  as  hydrogen  is  a  gas,  it  will  escape 
from  the  water  in  bubbles.  Now  there  are  such 
substances  —  substances  wherewith  oxygen  unites 
especially  willingly.  The  metal  sodium  is  one  of 
these.  This  metal  is  a  constituent  of  common  salt, 
which  consists  of  sodium  and  chlorine.  We  shall  not 
stop  here  to  examine  the  process  whereby  sodium  is 
obtained  by  common  salt;  that  will  came  later:  at 
present  we  want  this  metal  only  as  a  means  to  an  end. 

Sodium  has  so  great  an  affinity  for  oxygen  that  it  soon  unites 
with  that  element  (which  is  a  constituent  of  the  atmosphere)  if 
the  metal  is  left  exposed  to  the  air.  It  must  be  kept,  therefore, 
under  some  liquid  which  does  not  contain  oxygen  and  will 
prevent  the  oxygen  of  the  air  getting  at  the  sodium.  Paraffin  oil 
is  usually  employed  for  this  purpose  (see  fig.  12). 

To  prepare  hydrogen  gas  by  the  use  of  sodium,  we 
wrap  a  small  piece  of  the  metal,  taken  from  under 
paraffin  oil,  in  wire-gauze,  fasten  this  to  the  end  of 
a  rod  (A,  fig.  12),  and  bring  it,  in  the  manner  shown 
in  fig.  12,  under  a  cylinder  filled  with  water  and 
standing  on  the  shelf  of  a  pneumatic  trough.  Gas- 
bubbles  at  once  begin  to  rise  in  the  cylinder,  and 
after  a  short  time  the  cylinder  is  quite  filled  with  gas. 
This  gas  is  hydrogen.  We  now  slip  a  glass  plate  over 


30       INTRODUCTION    TO   MODERN    CHEMISTRY. 


the  mouth  of  the  cylinder  under  the  water,  and  reverse 
the  cylinder,  just  as  we  did  when  we  filled  a  cylinder 
with  oxygen- (see  p.  17);  then  we  withdraw  the  glass 
plate,  and  at  once  bring  a  lighted  taper  to  the  mouth  of 
the  cylinder.  The  taper  sets  fire  to  the  contents  of  the 
cylinder — that  is,  to  the  gas  we  have  obtained  from 

water  —  and    we 
see  the  gas  burn- 
ing with  a  feebly 
luminous       flame 
metal   sodium    is, 
then,  able  to  decompose 
cold  water  into  its  con- 
stituents, a  fact   which 
is    certainly    somewhat 
remarkable. 

We  must  not  pass 
over  the  other  constituent 
of  water,  oxygen  gas.  In  the 
present  experiment  the  oxygen 
of  the  water  unites  with  the 
sodium  to  form  a  compound 
which  we  know  will  be  called 
sodium  oxide.  As  that  com- 
pound dissolves  in  the  water,  we  have  no  direct  visible 
proof  of  its  production  in  this  experiment.  To  obtain 
the  compound  it  would  only  be  necessary  to  evaporate 
the  solution  of  it  in  water  to  dryness ;  but  to  do  that 
is  not  our  task  at  present. 

The  manipulation  of  sodium  is  not  particularly  con- 
venient.    Many   metals    that    are    known    to    us    in 


Fig.  12. — Production  of  hydrogen 
by  reaction  of  sodium  with 
water. 


PREPARATION    OF   HYDROGEN.  31 

ordinary  life  decompose  hot  water  in  the  same  way  as 
we  have  seen  sodium  decompose  cold  water.  Hydrogen 
was  first  obtained  from  water  by  passing  water-vapour 
over  red  hot  iron  :  under  these  conditions  iron  com- 
bines with  the  oxygen  of  the  water  to  form  iron  oxide, 
and  the  hydrogen  is  set  free.  This  experiment  must 
be  regarded  as  of  fundamental  importance;  for  the 
consideration  of  it  teaches  how  greatly  the  way  of 
looking  at  things  of  those  who  investigate  nature,  and 


Fig.  13.  -Preparation  of  hydrogen  by  passing  water-vapour  over  red  hot  iron. 

indeed  of  the  whole  race,  must  have  changed  after 
the  discovery,  made  in  the  last  quarter  of  the  eighteenth 
century  by  the  aid  of  this  experiment,  that  water— a 
substance  thought  of  for  a  thousand  years  as  something 
which  could  not  be  decomposed — was  formed  of  two 
constituents. 

In  conducting  this  experiment,  which  we  shall  do 
here  because  of  its  fundamental  significance,  we  pro- 
ceed as  follows.  We  fill  a  porcelain  tube  (A,  fig.  13) 


32        INTRODUCTION   TO   MODERN   CHEMISTRY. 

with  little  spirals  of  iron,  formed  by  twisting  thin  iron 
wire,  and  we  lay  the  tube  in  the  furnace  (B  in  the  figure) 
in  such  a  position  that  the  corks  in  the  ends  of  the 
tube  are  not  burnt  when  the  lamps  of  the  furnace  are 
lighted.  Water  is  heated  in  the  flask  G  ;  the  water- 
vapour  thus  produced  is  allowed  to  pass  through 
a  glass  tube  into  the  porcelain  tube  A,  wherein  the 
gaseous  water  comes  into  contact  with  the  red  hot  iron, 
which,  being  in  the  form  of  wire,  exposes  a  large  sur- 
face. Water  heated  above  100°  C.  [212°  F.]  is  gener- 
ally called  water-vapour ;  from  our  experiments  on  the 
distillation  of  water  we  know  that  this  vapour  is  a 
perfectly  transparent  air,  and  behaves  in  all  respects 
as  a  gas.  The  glowing  iron  seizes  the  oxygen  of  the 
water-vapour,  and,  combining  with  it,  forms  an  oxide 
of  iron.  The  hydrogen  gas,  which  is  now  set  free, 
passes  through  the  glass  conducting-tube  D,  and  issues 
under  the  inverted  cylinder  c  (which  is  filled  with 
water  and  stands  in  the  pneumatic  trough),  wherein 
it  collects.  When  the  cylinder  is  filled  with  the  gas, 
we  convince  ourselves,  by  the  same  means  as  before, 
that  we  have  to  do  with  a  combustible  gas,  produced 
from  the  water ;  this  gas,  as  we  know,  is  called 
hydrogen. 

Now  that  these  two  methods  of  preparing  hydrogen 
have  brought  home  to  us  the  existence  of  this  gas  in 
water  without  giving  us  visible  demonstration  of  the 
oxygen  which  forms  a  constituent  of  water,  we  are 
in  a  position  to  conduct  the  decomposition  of  water 
in  such  a  way  as  to  obtain  the  two  gases.  We  shall 
do  this  with  the  help  of  electricity.  The  electric  current 


ELECTROLYSIS   OF   WATER. 


33 


is  able  to  decompose  compounds  of  various  kinds, 
provided  they  conduct  electricity,  into  their  elements : 
it  splits  water  into  hydrogen  and  oxygen.  The  appa- 
ratus we  require  is  very  simple.  The  poles  of  a  galvanic 
battery  (A,  fig.  14)  are  arranged  in  the  water  of  a 


Fig.  14.— Electrolytic  decomposition  of  water  into  hydrogen  and  oxygen. 

pneumatic  trough  so  that  they  come  beneath  the 
cylinders  H  and  o,  which  are  filled  with  water.  In 
this  experiment  the  trough  consists  of  a  very  short- 
stemmed  funnel,  stopped  by  a  cork  through  which 
pass  the  conducting  wires  to  the  two  poles.  As  soon 
as  we  cause  the  current  to  pass  we  notice  bubbles  of 
gas  rising  from  the  two  poles.  The  hydrogen  gas  (H) 

3 


34        INTRODUCTION    TO    MODERN    CHEMISTRY. 

collects  at  the  negative  pole,  and  the  oxygen  gas  (o) 
at  the  positive  pole ;  and  we  notice  that  the  volume  of 
the  former  is  greater  than  that  of  the  latter.  Accurate 
investigations  have  shown  that  the  volume  of  the 
hydrogen  gas  is  double  that  of  the  oxygen  gas. 

There  is  another  method  for  obtaining  hydrogen  gas, 
which  is  much  more  convenient  than  those  we  have 
used,  and  for  that  reason  is  generally  employed  in 
the  laboratory.  That  method  we  shall  now  consider. 

There  are  many  metals  which  cause  the  production 
of  hydrogen  from  a  mixture  of  an  acid  with  water 
when  such  a  mixture  is  merely  poured  on  them.  In 
these  reactions  there  is  no  need  either  to  heat  the 
metal  or  to  use  an  electric  battery.  As  we  do  not 
yet  know  what  acids  are,  we  must  accept  this  method 
without  going  into  details  at  present :  it  is  for  this 
reason  that  the  process  has  been  considered  after 
the  others.  We  shall  soon,  however,  gain  an  insight 
into  the  chemical  bearings  of  the  reaction.  To  carry 
out  the  preparation  of  hydrogen  by  this  method,  we 
place  the  metal — zinc  filings,  for  example — in  a  flask 
(A,  fig.  15),  and  we  close  the  flask  with  a  cork  bored 
with  two  holes,  through  one  of  which  passes  a  long- 
stemmed  funnel  (B,  fig.  1 5.),  and  through  the  other  a 
bent  glass  tube  (D,  fig.  15),  which  serves  to  lead  the 
gas  from  the  flask  to  a  cylinder  (c)  in  the  pneumatic 
trough.  As  soon  as  the  apparatus  is  arranged  we 
pour  diluted  sulphuric  acid  through  the  funnel-tube  B 
on  to  the  zinc  in  the  flask,  and  without  more  ado 
we  obtain  a  stream  of  hydrogen  gas.  To  prevent 
the  gas  escaping  through  the  funnel-tube  instead  of 


COLLECTION    OF   HYDROGEN.  35 

streaming  into  the  cylinder,  we  must,  of  course,  pour 
so  much  diluted  sulphuric  acid  into  the  flask  that 
the  lower  end  of  the  funnel-tube  is  under  the  surface 
of  the  liquid  and  is  thereby  closed. 

By  employing  a  large  and  suitable  apparatus  this 
method  enables  us  to  have  always  at  hand  a  means 
of  obtaining  a  stream  of  hydrogen.  The  stream  will 


Fig.  15. — Collection  of  hydrogen  gas  over  water. 

last,  of  course,  only  until  the  sulphuric  acid  or  the 
zinc  is  used  up,  but  that  will  be  a  considerable  time. 
The  apparatus  generally  employed  for  this  purpose  is 
that  first  proposed  by  Kipp.  It  consists  of  two  parts — 
a  funnel  with  a  spherical  widening  (A,  fig.  16),  and 
a  lower  part  into  which  the  funnel  is  ground  air-tight 
(at  B,  fig.  1 6).  Fig.  1 6  shows  the  separate  parts  of 
the  apparatus  both  when  empty  and  when  filled.  The 
lower  part  consists  of  the  bulb  c,  which  is  connected 
with  the  space  D  by  the  contraction  v.  The  bulb  c 


36       INTRODUCTION   TO   MODERN   CHEMISTRY. 

has  an  opening  (E) — such  an  opening  is  called  a  tubulus 
— closed  by  a  cork  through  which  passes  a  glass  tube 
furnished  with  a  stopcock  (F).  In  using  this  apparatus 
for  making  hydrogen  gas,  the  funnel  A  is  set  in  the 
lower  part,  the  bulb  c  is  about  half  filled,  through  E, 
with  pieces  of  zinc,  which  are  prevented  from  falling 
into  the  lowest  part  D  by  the  contraction  v,  and  the 
tubulus  is  closed  by  pressing  into  it  the  cork  with  the 


Fig  16. — Kipp's  apparatus. 

stopcock  F.  Diluted  sulphuric  acid  is  then  poured  into 
the  funnel  A,  through  which  it  passes  into  the  lower 
vessel  D.  As  long  as  the  stopcock  F  is  kept  closed 
the  acid  cannot  either  fill  D  or  rise  into  c,  and  therefore 
cannot  come  into  contact  with  the  zinc,  because  it  is 
prevented  by  the  air  in  the  lower  vessel  and  in  the 
bulb;  but  as  soon  as  the  stopcock  F  is  opened  the 
air  escapes  from  c  and  D,  driven  out  by  the  acid  falling 
down  from  A,  and  the  acid  rises  through  v  until  it 
reaches  the  zinc.  Immediately  the  acid  and  the  zinc 


PROPERTIES  OF  HYDROGEN.         37 

are  in  contact  the  evolution  of  hydrogen  gas  begins. 
When  no  more  hydrogen  is  required  the  stopcock  F 
is  closed.  As  the  acid  and  the  zinc  are  still  in  contact, 
hydrogen  continues  to  be  produced ;  but  as  the  gas 
cannot  escape  from  c,  it  fills  the  bulb,  and  its  pressure 
automatically  drives  the  acid  out  of  c  and  so  removes 
it  from  the  zinc.  Part  of  the  acid  finds  its  way  into 
the  lower  vessel  D  and  part  of  it  into  the  funnel  A. 
The  evolution  of  gas  ceases,  to  begin  again  when 
the  stopcock  is  opened  and  the  acid  is  thereby  allowed 
to  reach  the  zinc.  In  a  word,  an  apparatus  of  this 
kind  is  a  convenient  and  portable  hydrogen  manufactory. 

The  extraordinary  lightness  of  hydrogen  gas  may 
be  demonstrated  by  the  use  of  a  small  balloon  made 
of  collodion.  Such  a  balloon  is  placed  on  the  end 
of  the  glass  tube  leading  from  the  Kipp's  apparatus, 
the  stopcock  is  opened,  and  so  the  balloon  is  filled 
with  hydrogen.  On  removing  the  balloon  from  the 
apparatus,  it  rises  like  an  "  air-balloon  "  to  the  ceiling 
of  the  room  ;  hence  the  balloon  and  its  contents  must 
be  much  lighter  than  the  air  which  surrounds  us. 

i 

Hydrogen  is  a  colourless  and  odourless  gas.  When 
ignited  it  is  burnt  to  water.  The  quickest  way  of 
proving  this  is  to  allow  a  hydrogen  flame  to  burn  within 
a  large  glass  bell-jar,  in  the  manner  shown  in  fig.  17. 
In  a  very  short  space  of  time  we  see  that  the  inside 
of  the  jar  is  moistened,  and  it  is  not  long  before  drops 
of  water  trickle  from  the  open  end.  This  formation 
of  water  can  be  caused  only  by  the  flame  which  is 


38        INTRODUCTION    TO   MODERN    CHEMISTRY. 


burning    in   the   jar;    hence    the    hydrogen    must   be 
burning  to  water. 

If  hydrogen  is  mixed   with  air,   we  get  a  mixture 
which  burns,  when  ignited,  with  a  sharp  explosion.    To 

show  this  we  fill  a 
cylinder  with  water,  in- 
vert it  in  the  pneumatic 
trough,  and  allow  hydro- 
gen, from  a  Kipp's  ap- 
paratus, to  pass  into  it 
until  it  is  half  filled  with 
that  gas;  we  then  raise 
the  cylinder  a  little  out 
of  the  water,  so  that  the 
air  enters  and  fills  it. 
When  a  light  is  now 
brought  to  the  mouth 
of  the  cylinder,  the 
mixture  of  hydrogen  and 
air  ignites  with  a  loud 
detonation. 

If  pure  oxygen  is  used 
in  place  of  air,  we  get 
a  mixture  which  explodes  with  great  violence  when 
ignited  :  this  mixture  is  known  as  explosive  gas.  It 
is,  of  course,  the  sudden  combination  of  the  whole 
of  the  gases  in  the  cylinder  to  form  water  that  causes 
the  explosion.  When  a  cylinder  is  filled  with  hydrogen 
and  the  gas  is  ignited,  combination  can  take  place 
only  in  so  far  as  the  outer  air,  which  supplies  the 
oxygen,  is  able  to  come  into  contact  with  the  hydrogen. 


Fig.  17. — Formation  of  water  by  burning 
hydrogen. 


PROPERTIES  OF  HYDROGEN.         39 

We  can  scarcely  say  anything  more  about  hydrogen 
at  present,  because  of  the  other  elements  wherewith 
it  forms  compounds  we  know  nothing  except  the 
names.  In  the  course  of  this  INTRODUCTION  we  shall, 
however,  very  often  come  back  to  hydrogen,  because 
we  shall  have  to  speak  of  its  compounds  with  other 
elements — compounds  of  the  greatest  importance — 
when  we  have  learned  something  about  these  elements. 
The  properties  of  hydrogen  mark  it  off  from  the  other 
elements,  so  that  it  stands  in  a  somewhat  isolated 
position. 

We  now  proceed  to  a  group  of  four  elements  which 
are  very  much  alike ;  these  are  the  elements  chlorine, 
bromine,  iodine,  and  fluorine. 


CHLORINE,  BROMINE,  IODINE,  FLUORINE, 

AND  THEIR  COMPOUNDS  WITH  HYDROGEN. 

CHLORINE,  like  hydrogen,  is  a  gas.  Just  as  we 
often  omit  the  word  gas  when  speaking  of  hydrogen 
or  oxygen,  so  we  do  not  always  say  chlorine  gas,  but 
only  chlorine. 

To  prepare  chlorine  gas  we  use  the  reaction  between 
hydrochloric  acid  and  pyrolusite.  These  two  sub- 
stances are  strange  to  us  as  yet.  The  first,  hydro- 
chloric acid,  is  a  compound  of  hydrogen  with  chlorine, 
and  we  should  therefore  expect  it  to  be  called  hydrogen 
chloride.  We  shall  soon  discover  why  it  is  spoken 
of  as  an  acid.  The  second  substance,  pyrolusite,  is 
a  mineral,  quantities  of  which  are  found  in  nature. 
Because  of  its  chemical  composition  it  is  called  man- 
ganese peroxide.  Manganese  is  a  metal  resembling 
iron.  As  we  may  easily  understand,  the  word  peroxide 
implies  that  this  compound  is  a  body  rich  in  oxygen. 
We  already  know  that  oxygen  very  readily  combines 
with  hydrogen  to  form  water:  the  oxygen  of  man- 
ganese peroxide  reacts  with  hydrochloric  acid  in 
such  a  way  that  it  combines  with  the  hydrogen  of 
that  compound  to  form  water,  while  the  chlorine  is 
set  free. 


PREPARATION   OF   CHLORINE.  41 

We  shall  become  acquainted  later  with  the  equation — that  is, 
the  precise  statement  in  chemical  language— which  expresses  the 
reaction  whereby  chlorine  gas  is  produced.  That  equation  will 
also  show  us  what  becomes  of  the  manganese  in  the  reaction. 

Chlorine  was  discovered  in  1774.  The  name  chlorine 
(from  the  Greek  word  ^Xa)p6<;  =  yellow-green)  was 
given  to  it  because  of  its  characteristic  colour.  The 
gas  has  a  peculiar,  choking  smell ;  it  acts  on  almost 
everything  wherewith  it  comes  into  contact.  The 
action  of  chlorine  on  the  lungs,  for  instance,  is  very 
great ;  if  it  is  breathed  it  soon  causes  bl&od-spitting  ; 
the  gas  must  therefore  be  handled  with  the  greatest 
care.  Chlorine  is  fairly  soluble  in  water,  so  that  if  we 
tried  to  collect  it  in  a  cylinder  over  water,  as  we 
collected  hydrogen  and  oxygen,  we  should  obtain 
scarcely  any  gas  in  the  cylinder  ;  for,  as  the  gas  is 
dissolved  by  water,  it  would  not  appear  in  the  cylinder, 
but  would  be  swallowed  by  the  water.  That  gases 
should  be  dissolved  by  water  can  appear  strange  only 
for  a  moment ;  for  why  should  not  water,  that  dissolves 
so  many  things,  such  as  salt,  sugar,  and  the  like,  not 
dissolve  gases  also  which  are  passed  through  it  ?  A 
solution  of  chlorine  in  water  is  called  chlorine  water. 

In  order  to  prevent  injury  to  health,  experiments 
with  chlorine  should  be  conducted  in  a  draught- 
chamber.  A  draught-chamber  is  an  arrangement  found 
in  all  laboratories ;  it  is  used  to  prevent  accidents  when 
working  with  poisonous  gases  or  vapours.  It  is  a 
cupboard  with  glass  walls,  the  back  whereof  is  the  wall 
of  the  room  (see  fig.  1 8).  A  pipe  leads  through  the 
wall  to  the  open  air  in  the  manner  of  a  chimney ;  this 


42        INTRODUCTION    TO   MODERN    CHEMISTRY. 

pipe  should  be  carried  above  the  roof  of  the  building. 
If  a  flame  is  burnt  in  this  pipe  (every  draught-chamber 


Fig.  18. — Draught-chamber  for  working  with  poisonous  gases  and  badly 
smelling  substances. 

is  furnished  with  a  suitably  arranged  gas-jet ;   see  A, 
fig.   1 8),  all   hurtful  gases  and   vapours  that  may  be 


PREPARATION    OF   CHLORINE.  43 

produced  in  the  chamber  are  drawn  up  the  pipe  and  are 
discharged  into  the  open  air  above  the  roof  of  the 
building.  If  the  front  sash  of  the  chamber  (B,  fig.  18) 
is  kept  slightly  open,  the  draught  will  be  so  strong  that 
no  inconvenience  will  be  experienced  in  the  working- 
room  when  experiments  with  badly  smelling  gases  are 
carried  on  in  the  chamber.  A  draught-chamber  should 


Fig.  19.— Collection  of  chlorine  gas  by  making  use  of  its  high  specific 
gravity. 

always  be  furnished  with   gas   and  water  taps,  as  is 
shown  in  the  figure. 

The  apparatus  that  is  required  for  preparing  chlorine 
is  shown  in  fig.  19 ;  this  apparatus  must  be  set 
up  in  the  draught-chamber.  Pyrolusite  (manganese 
peroxide)  is  placed  in  the  flask  c;  sufficient  hydrochloric 
acid  to  cover  the  pyrolusite  is  added,  and  the  flask  is 
warmed.  A  yellowish  gas  very  soon  begins  to  come 


44       INTRODUCTION    TO   MODERN   CHEMISTRY. 

off  from  the  mixture  ;  this  gas  gradually  drives  out  the 
air  and  fills  the  whole  apparatus.  The  method  which 
is  employed  for  filling  vessels  with  hydrogen  and  most 
of  the  other  gases,  by  using  a  pneumatic  trough,  cannot 
be  applied  in  the  case  of  chlorine.  When  we  wish  to  fill 
a  vessel  with  chlorine  we  take  advantage  of  the  fact 
that  this  gas  is  much  heavier  than  air.  Chlorine  gas  is 
2*45  times  heavier  than  air.  If,  then,  we  lead  chlorine 
gas  to  the  bottom  of  a  cylinder  (A,  fig.  19),  the  cylinder 
will  be  gradually  filled  with  the  gas  ;  for  the  heavy 
chlorine,  seeking  to  remain  in  the  lower  part  of  the 
cylinder,  drives  out  the  air.  If  the  mouth  of  the 
cylinder  were  open,  it  would  not  be  possible  to  pre- 
vent the  admixture  of  some  air  with  the  chlorine  in 
the  upper  part  of  the  cylinder.  In  order  to  ensure  the 
complete  filling  of  the  cylinder  with  chlorine,  we  place 
a  cork,  bored  with  two  holes,  in  the  mouth  of  the 
cylinder,  and  through  one  of  these  holes  we  pass  a  tube 
(D,  fig.  19),  which  conducts  the  chlorine  to  the  bottom 
of  the  cylinder,  while  through  the  other  hole  passes  a 
short  piece  of  glass  tube  (E  in  the  figure),  cut  flush  with 
the  under  surface  of  the  cork,  which  allows  the  exit  of 
the  air  from  the  cylinder.  The  air  is  thusx -gradually  but 
completely  driven  out  by  the  chlorine  gas.  When  pure 
chlorine  is  issuing  from  the  tube  E  we  know  that  the 
cylinder  is  quite  full  of  that  gas. 

Fig.  19  shows  a  flask  (B),  containing  some  liquid, 
placed  before  the  cylinder  which  we  wish  to  fill  with 
chlorine.  As  the  tube  which  conducts  the  chlorine 
passes  under  the  surface  of  the  liquid  in  the  flask  B, 
and  nearly  to  the  bottom  of  that  flask,  the  stream  of 


WASHING   AND  DRYING  GASES.  45 

chlorine  must  bubble  through  the  liquid  in  B.  Such  a 
flask,  inserted  in  an  apparatus  for  producing  a  gas,  is 
called  a  washing-flask  (sometimes  a  wash-bottle).  In 
the  present  case  the  washing-flask  contains  water ;  this 
catches  and  retains  any  traces  of  hydrochloric  acid  that 
may  be  carried  over  from  the  vessel  wherein  the 
chlorine  is  produced. 

Flasks  of  this  kind  can  be  used  not  only  for  washing, 
but  also  for  drying,  gases  ;  for  gases  which  are  evolved 
from  mixtures  that  contain  water  are  moist,  just  as 
the  air  that  surrounds  us  is  moist  because  it  is  so 
often  in  contact  with  water.  There  are  certain 
substances  which  have  so  strong  an  attraction  for 
moisture  that  they  take  away,  and  retain,  or  hold 
fast,  all  the  moisture  from  a  gas  passing  through  them. 
Sulphuric  acid,  which  we  shall  examine  in  detail  at 
a  later  time,  is  one  of  those  substances  which  have 
a  great  attraction  for  water.  Besides  rendering  this 
service,  which  is  often  of  great  importance,  washing- 
flasks  enable  us  to  determine  whether  the  evolution 
of  a  gas  from  the  materials  that  produce  it  is  proceed- 
ing too  quickly  or  too  slowly,  and  therefore  whether 
it  is  necessary  to  slacken  or  to  quicken  the  reaction  : 
we  have  only  to  watch  the  rate  whereat  the  bubbles 
of  gas  pass  through  the  liquid  in  the  washing-flask. 
For  this  reason,  if  for  no  other,  almost  every  gas- 
evolving  apparatus  is  furnished  with  a  washing-flask. 

We  shall  now  use  the  cylinders  we  have  filled  with 
chlorine,  in  the  way  already  described,  for  a  series 
of  experiments. 

Into  the  first   cylinder   we   shake    some   antimony. 


46        INTRODUCTION    TO   MODERN    CHEMISTRY. 


Fig.  20.— Antimony 
burning    in    chlorine. 


Antimony  is  a  metal-like  element;  it  is  so  brittle 
that  it  can  be  powdered.  Fig.  20  represents  the 
antimony  falling  into  the  chlorine  in  a  cylinder  and 
burning  therein.  The  two  ele- 
ments chlorine  and  antimony  com- 
bine, and  antimony  chloride  is 
produced.  The  striving  to  combine 
of  the  elements  is  so  great  that  the 
act  of  combination  is  accompanied 
by  the  glowing  of  the  materials 
and  the  phenomena  of  fire. 

Into  the  second  cylinder  we  lower 
a  small  lighted  candle  fastened   to 
the  end  of  a  wire  (see  fig.  21).     At 
once   the  flame,  which    was   white, 
becomes  red,  and,  besides  this,  the  n 

cylinder  gets  filled  with  soot.  This 
is  due  to  the  action  of  the  chlorine 
on  the  compounds  of  the  two 
elements  carbon  and  hydrogen, 
called  hydrocarbons,  which  are  pre- 
sent in  such  a  flame.  The  chlorine 
combines  with  the  hydrogen  of 
these  compounds  to  form  hydro- 
chloric acid  (we  know  that  chlorine 
and  hydrogen  have  a  great  tend- 
ency to  combine  with  one  another); 
hence  the  carbon  must  separate 
from  the  hydrocarbons.  We  see 
this  carbon,  in  the  form  of  soot,  filling  the  cylinder. 
In  these  experiments  we  have  two  examples  of  the 
violent  way  wherein  chlorine  attacks  various  substances. 


.<* 


Fig.  21. — A  candle  burn- 
ing in  chlorine. 


PROPERTIES   OF   CHLORINE.  47 

In  one  case  chlorine  combined  with  another  element 
with  the  production  of  fire  ;  in  the  other  case  it  com- 
pletely changed  the  character  of  a  luminous  flame. 

The  great  striving  to  combine  that  there  is  between 
chlorine  and  hydrogen,  which  is  the  cause  of  the 
production  of  soot  from  a  candle  burning  in  chlorine, 
may  be  enforced  directly.  If  a  mixture  of  these  two 
gases,  prepared  by  filling  a  cylinder  half  with  chlorine 
and  half  with  hydrogen,  is  placed  in  ordinary  day- 
light, the  gases  combine  gradually  to  form  hydrochloric 
acid  ;  but  if  a  direct  ray  of  sunlight  falls  on  the  mixture, 
the  gases  combine  instantaneously  (of  course,  to  form 
hydrochloric  acid)  and  with  a  very  violent  explosion. 

If  some  solution  of  indigo  is  poured  into  a  third 
cylinder  filled  with  chlorine,  the  indigo  is  at  once 
decolourised.  Pieces  of  coloured  cloths  fare  no  better ; 
they  are  bleached  in  a  very  short  time.  Because  of 
its  chemical  energy,  chlorine  rapidly  destroys  colours  of 
many  sorts.  Very  wide  use  is  made  of  this  property 
of  chlorine ;  for  instance,  it  is  applied,  as  is  well 
known,  to  bleaching  clothes,  material  for  making  paper, 
and  to  many  other  industrial  purposes.  We  have  seen 
that  it  is  not  exactly  agreeable  to  work  with  a  sub- 
stance so  destructive  of  health  as  chlorine,  even  in  a 
chemical  laboratory.  The'  use  of  chlorine  in  mills, 
paper  factories,  and  the  like  would  scarcely  be  possible. 
The  difficulty  has  been  got  over  in  the  following  way. 
Chlorine  is  passed  over  slaked  lime  in  specially  con- 
structed chemical  factories.  A  substance  called  bleaching 
powder,  which  is  known  by  everyone  to  have  strong 
bleaching  power,  is  thus  produced.  Bleaching  powder 


48       INTRODUCTION   TO   MODERN    CHEMISTRY. 

can  be  bought  for  the  purpose  of  any  industry  wherein 
bleaching  is  practised,  so  that  it  is  not  necessary  for 
the  manufacturer  to  concern  himself  with  the  preparation 
of  chlorine. 

Let  us  make  some  bleaching  powder.  To  do  this 
it  is  only  necessary  to  arrange  an  apparatus  as  shown 
in  fig.  22,  and  to  allow  chlorine,  made  from  manganese 
peroxide  and  hydrochloric  acid,  and  dried  by  passing 
through  sulphuric  acid  in  the  washing-flask  w,  to  pass 
over  slaked  lime  placed  in  the  long  tube  A.  The  large 
surface  of  lime  exposed  to  the  action  of  the  chlorine 
hastens  the  saturation  of  the  lime  with  that  gas.  After 
some  time  the  lime  is  changed  into  bleaching  powder.* 

In  the  foregoing  experiments  the  cork  which  closes  the  flask 
wherein  the  chlorine  is  generated  is  bored  with  two  holes  ; 
through  one  of  these  passes  the  tube  which  leads  the  chlorine  to 
the  washing-flask  w,  and  through  the  other  a  safety  funnel  is 
fitted.  When  hydrochloric  acid  is  poured  through  this  funnel  a 
part  of  the  liquid  remains  in  the  looped  part  of  the  funnel,  as 
shown  in  the  figure  ;  hence  gas  cannot  escape  through  the 
safety  funnel,  provided  the  production  of  gas  in  the  flask  is  not 
so  rapid  that  the  whole  of  the  gas  cannot  be  carried  off  by  the 
proper  conducting  tube  (A  in  fig.  22).  Should  the  rush  of  gas  be 
too  rapid,  the  increasing  pressure  in  the  flask  wherein  the  reaction 
occurs  will  lift  the  liquid  plug  in  the  safety  funnel,  and  gas  will 
then  be  able  to  escape  through  that  funnel.  If  there  is  no  safety 
funnel,  the  apparatus  may  be  separated  into  pieces,  by  the 
blowing  of  the  cork  out  of  the  flask,  or  by  something  of  that 
kind.  Unskilled  experimenters  do  well  to  place  a  safety  funnel 
in  every  apparatus  for  making  a  gas. 

The  method  of  bleaching   by   means   of  bleaching 

*  Bleaching  powder  is  sometimes  called  chloride  of  lime.  It 
must  not  be  confused  with  another  substance  called  chloride  of 
calcium,  which  we  shall  become  acquainted  with  later. 


PREPARATION    OF    BLEACHING   POWDER.          49 

powder,  which  has  made  diverse  industries  independent 
of  bleaching  greens  and  sunshine,  leaves  nothing  to 
be  desired  in  the  way  of  convenience.  It  has  only 
one  drawback  :  the  chemical  energy  of  the  chloride 
of  lime  which  destroys  colours  is  not  exhausted  when 
the  colours  have  disappeared.  On  the  other  hand,  after 
it  has  bleached  the  colours — and  it  does  this  very 


Fig.  22.— Preparation  of  chloride  of  lime  (bleaching  powder). 

quickly — the  chloride  of  lime  begins  to  act  on  and 
to  destroy  the  fibres  to  which  the  colours  adhere. 
It  is  therefore  necessary,  immediately  the  colours 
have  vanished  and  the  bleaching  action  has  gone  far 
enough,  at  once  to  stop  the  further  action  of  the 
chloride  of  lime.  This  can  be  done  very  easily  by 
adding  one  of  several  chemicals,  all  of  which  are 
classed  together  under  the  name  "  antichlors"  These 

4 


50       INTRODUCTION    TO   MODERN   CHEMISTRY. 

substances  change  chloride  of  lime  into  the  perfectly 
harmless  compound  chloride  of  calcium — a  compound 
related  to,  but  very  different  from,  chloride  of  lime — 
while  the  substances  themselves  are  changed  to  others 
which  have  no  harmful  action  on  any  kinds  of  fibres. 
Thiosulphate  of  sodium,  commercially  known  as  hypo- 
sulphite of  soda,  is  the  favourite  antichlor :  we  shall 
learn  something  of  this  substance  when  we  come  to 
consider  sulphur  and  its  compounds.  We  shall  also 
defer  the  consideration  of  the  formula  of  chloride  of 
lime,  which  is  somewhat  complicated,  to  a  later  part 
of  the  book. 

Before  leaving  the  subject  of  chlorine  we  shall  make 
silver  chloride,  because  the  formation  of  this  compound, 
which  is  quite  insoluble  in  water,  is  used  by  chemists 
as  a  means  of  recognising  the  presence  of  chlorine, 
and  the  compound  is  therefore  of  especial  interest.  If 
a  solution  of  nitrate  of  silver  is  added  to  any  liquid 
containing  chlorine,  chloride  of  silver,  being  insoluble, 
separates  from  the  liquid.  In  this  way  it  is  easy  to 
make  certain  of  the  presence  of  chlorine  in  a  solution. 
To  observe  this  reaction,  all  we  have  to  do  is  to 
add  a  solution  of  silver  to,  say,  a  solution  of  common 
salt — that  is,  sodium  chloride.  Chloride  of  silver  at 
once  falls  down  as  a  white,  thick,  and  (as  it  is  called) 
curdy  precipitate ;  while  nitrate  of  sodium,  which  is  ' 
formed  by  the  reaction  of  the  two  substances  with 
one  another,  remains  in  the  solution.  The  chemical 
change  may  be  expressed  thus  :— 

Sodium  chloride  +  silver  nitrate  =  silver  chloride  +  sodium  nitrate, 
(soluble  in  water)  (soluble  in  water)  (insoluble  in  water  (soluble  in  water) 

and  so  precipitates) 


PREPARATION    OF   BROMINE. 


BROMINE. 

The  element  bromine  is  a  liquid  with  a  dark  red 
colour  and  a  most  unpleasant  smell.  Its  specific 
gravity  is  3-18  referred  to  water  as  unity.  The  Greek 
word  /3/OW//09  means  a  bad  smell,  and  the  discoverer 
of  the  element  derived  the  name  from  that  word. 
Bromine  is  exceedingly  like  chlorine  in  its  chemical 
behaviour.  We  can  prepare  bromine  by  a  method 
very  similar  to 
that  whereby  we 
prepared  chlorine. 
The  heating  of 
manganese  per- 
oxide with  hydro- 
chloric acid  gave 
us  chlorine ;  the  heating  of 
manganese  peroxide  with 
hydrobromic  acid  gives  us 
bromine.  As  in  the  former 
case  the  oxygen  of  the  per- 
oxide Seizes  the  hydrogen  FiS'  ^-Preparation  of  bromine. 

of  the  hydrochloric  acid,  so  in  the  latter  reaction  the 
oxygen  seizes  the  hydrogen  of  the  hydrobromic  acid 
and  bromine  is  set  free.  But  whereas  chlorine  is  a 
gas,,  bromine  is  a  liquid  :  therefore,  in  order  to  obtain 
bromine,  we  require,  not  an  apparatus  for  producing 
a  gas,  but  a  retort  and  a  receiver.  We  put  the  mix- 
ture of  hydrobromic  acid  and  .manganese  peroxide 
into  a  .retort  (A,  fig.  23),  heat  the  retort,  and  collect 
the  bromine  that  is  set  free  and  distils  over  in  a 
receiver  (B,  fig.  23).  To  guard  ourselves  from  the 


52        INTRODUCTION    TO    MODERN    CHEMISTRY. 

almost  unbearable  smell  of  bromine,  we  lead  away  any 
bromine  which  does  not  condense  in  the  receiver  by 
the  glass  tube  c  to  the  draught-chamber. 

Bromine  was  discovered  in  the  twenties  of  the 
nineteenth  century  in  the  mother-liquors  of  the  sea- 
salts  obtained  in  the  south  of  France.  The  word 
mother-liquor  should  be  explained  before  we  proceed 
farther.  The  explanation  may  run  somewhat  as  follows : 
— It  seems  to  us  self-evident,  but  only  because  we  have 
heard  nothing  to  the  contrary,  that  a  chemical  compound 
should  have  the  same  composition  wherever  it  may  be 
found.  No  one  doubts,  for  instance,  that  distilled  water 
will  always  be  the  same  substance,  whether  it  be  pre- 
pared by  distilling  the  water  of  the  Thames,  the  Orinoco, 
or  the  Ganges,  or  whether  it  be  obtained  from  sea-water 
on  board  a  ship  in  any  part  of  the  ocean.  In  the  same 
way,  it  seems  to  us  self-evident  that  common  salt,  which 
we  know  to  consist  of  sodium  and  chlorine,  must  always 
have  the  same  composition,  in  whatever  part  of  the  earth 
it  may  be  found.  A  cogent  proof  of  this  identity  is 
not,  however,  furnished  save  by  further  considerations. 
Nature  might  perfectly  well  have  allowed  slight  differ- 
ences in  the  compositions  of  chemical  compounds  in 
places  far  removed  from  one  another — American  oak,  for 
instance,  differs  from  European  oak.  But  we  shall  see 
that,  according  to  the  atomic  theory,  with  which  we  shall 
soon  become  acquainted — that  is,  according  to  the  view 
that  the  elements  are  composed  of  atoms,  a  view  which 
sets  forth  all  the  phenomena  in  the  domain  of  chemistry, 
without  exception,  in  a  very  wonderful  way — every 
chemical  compound  must  always  have  exactly  the  same 


CRYSTALLISATION.  53 

composition.  The  constituents  of  complicated  mixtures 
of  chemical  compounds  of  all  kinds,  such  as  that  which 
in  its  totality  represents  the  thing  we  call  wood,  may, 
of  course,  be  put  together  in  the  most  different  pro- 
portions. But  every  single  chemical  compound  which 
is  found  in  wood — cellulose,  for  example — from  whatever 
kind  of  wood  it  may  be  prepared,  is  always  composed 
of  the  same  elements  united  in  the  same  proportionate 
quantities. 

It  is  much  less  generally  known  that,  besides  giving 
to  every  chemical  compound  a  composition  which  is 
unchangeable  for  all  time,  nature  has  bestowed  a 
determinate  form  on  every  compound  that  appears 
as  a  solid.  Everyone  knows  that  formless  water 
changes  into  six-sided  star-like  shapes  at  the  moment 
when  it  becomes  snow.  This  configuration  of  water 
which  remains  for  ever  the  same  is  also  to  be  recognised 
in  ice.  This  shape  is  designated  the  crystalline  form 
of  the  compound.  Mixtures  of  any  kind — wood,  for 
instance — have  no  crystalline  forms.  In  this  respect 
there  is  an  essential  difference  between  mixtures  and 
chemical  compounds.  That  solid  compounds  may  as- 
sume their  proper  crystalline  form — and  this  is  a  point 
of  special  importance—  nature  has  impressed  on  crystals, 
while  they  are  forming,  a  tendency  to  crystallise  from 
their  solutions  in  as  pure  a  state  as  possible.  Hence 
it  is  that  crystallising  out  is  a  method  of  purification 
constantly  used,  both  by  the  chemist  in  his  laboratory 
and  by  the  manufacturer  in  his  chemical  industry ;  in 
numberless  cases,  indeed,  this  is  the  only  method  where- 
by the  wished-for  result  can  be  gained. 


54        INTRODUCTION    TO    MODERN    CHEMISTRY. 

The  following  experiment  will  make  clear  to  us  the 
mode  and  manner  of  applying  this  process  to  the  purifi- 
cation of  chemical  compounds — that  is,  to  the  freeing  of 
them  from  all  impurities  that  are  mixed  with  them.  In 
carrying  out  a  process  of  crystallisation  we  shall 
select  a  substance  which  will  crystallise  so  quickly  that 
we  shall  be  able  to  realise  the  result  of  our  work  in  the 
shortest  time  possible.  Benzoic  acid  is  very  suitable 


Fig.  24. — Filtration  of  a  boiling  liquid  from  which  a  solid  is  to  be 
crystallised. 

for  our  purpose ;  we  shall  therefore  choose  that  sub- 
stance. Benzoic  acid  appears  in  commerce  as  a  white 
glistening  powder;  we  shall  first  of  all  intentionally 
make  some  of  this  powder  impure,  and  then  separate  it 
from  these  impurities  by  crystallisation.  For  this 
purpose  we  mix  some  benzoic  acid  with  sand  and  a 
little  common  salt  ;  then  we  throw  our  mixture  into 
water  which  is  boiling  in  a  beaker  (A,  fig.  24).  The 
benzoic  acid  and  the  common  salt  dissolve  in  the  hot 
water ;  the  sand,  of  course,  does  not  dissolve.  To  get  rid 


SEPARATION    BY   CRYSTALLISING.  55 

of  the  sand  we  run  the  liquor  through  a  filter  made  of 
paper,  placed  in  the  funnel  B  (fig.  24).  The  sand  cannot 
pass  through  the  filter ;  it  remains  on  the  paper.  In 
this  way  we  have  separated  the  sand  from  the  benzole 
acid  which  is  found  in  the  filtrate.  As  the  warm  and 
perfectly  clear  filtrate,  which  runs  through  the  filter 
and  collects  in  the  beaker  c  (fig.  24),  cools,  it  becomes 
filled  with  glistening  crystals.  These  are  crystals  of 
benzoic  acid  :  this  compound  is  scarcely  soluble  in  cold 
water,  although  it  dissolves  readily  enough  in  hot  water; 
hence,  as  the  solution  cools,  the  acid  crystallises  out  in 
that  special  crystalline  form  which  has  been  impressed 
on  it  by  nature.  The  small  quantity  of  common  salt 
that  we  mixed  with  the  benzoic  acid  remains  dissolved 
in  the  cold  liquid,  so  that  the  benzoic  acid  which  has 
crystallised  out  is  free  from  that  impurity.  The 
liquid  which  is  above  the  crystals  of  benzoic  acid  is  the 
mother-liquor  from  these  crystals.  The  name  mother- 
liquor  is  applied  to  any  liquid  from  which  something  has 
crystallised  out.  In  the  olden  days  such  a  liquid  was 
thought  of  as  the  mother  of  the  crystals  that  separated 
from  it;  the  name  derived  from  this  conception  has 
remained  until  now.  If  we  throw  the  mother-liquor 
and  the  crystals  of  benzoic  acid  therein  on  to  a  filter,  the 
benzoic  acid  remains  on  the  filter,  and  the  filtrate — that 
is,  the  mother-liquor — which  contains  the  common  salt, 
runs  through.  Should  we  be  afraid  that  the  benzoic 
acid  on  the  filter  may  have  a  little  salt  adhering  to 
it,  we  may,  of  course,  repeat  the  whole  process  :  we 
dissolve  the  solid  in  a  fresh  quantity  of  hot  water  and 
proceed  as  before,  and  in  this  way  we  purify  our  benzoic 
acid  by  re-cry stallisation}  as  this  process  is  called.  The 


56        INTRODUCTION    TO    MODERN    CHEMISTRY. 

traces  of  salt  which  may  have  adhered  to  the  once- 
crystallised  benzole  acid  are  certain  to  remain  in  the 
mother-liquor  after  the  second  crystallisation,  and  the 
benzoic  acid  obtained  by  this  repetition  of  the  process 
of  crystallising  .is  undoubtedly  free  from  common  salt. 
It  is  only  necessary  to  throw  the  crystals  on  to  a  filter, 
to  allow  the  mother-liquor  to  drain  away,  and  to  dry 
the  solid  that  remains,  in  order  to  obtain  pure  benzoic 
acid.  In  difficult  cases  one  may  have  to  crystallise 
thirty  times,  or  even  more,  in  order  to  free  a  sub- 
stance from  all  impurities  wherewith  it  may  be  mixed. 
These  impurities  remain  at  last  in  the  mother-liquors. 

The  method  we  have  illustrated  by  our  experiment 
with  benzoic  acid  is  employed,  not  only  in  laboratories, 
but  also  in  manufactures.  To  take  an  example.  There 
are  large  districts  in  Chili  where  soda  saltpetre — or 
Chili  saltpetre,  as  it  is  often  called — is  found.  When 
we  come  to  speak  of  artificial  manures  we  shall  meet 
this  substance  again  ;  at  present  we  are  using  it  only  as 
an  illustration.  Chili  saltpetre  is  very  soluble  in  hot 
water ;  if,  therefore,  some  of  the  soil  wherein  this  com- 
pound is  found  is  boiled  with  water,  the  saltpetre  is 
dissolved  in  the  water.  If  the  hot  solution  is  then 
separated  from  the  undissolved  soil,  in  the  manner 
whereby  we  separated  the  sand  intentionally  added  to 
benzoic  acid  from  the  benzoic  acid,  the  saltpetre  will 
crystallise  out  from  the  filtrate  as  that  cools!  The 
crystallisation  will  proceed  in  this  case  much  more 
slowly  than  in  the  case  of  benzoic  acid,  and  some 
days  may  elapse  before  the  process  is  completed. 
We  shall  very  soon  have  to  concern  ourselves  with 


SOURCES   OF    BROMINE.  57 

the  liquid  from  which  Chili  saltpetre  crystallises — 
that  is,  with  the  mother-liquor  of  that  substance  — 
because  that  liquid  contains  iodine ;  and  this  is  another 
reason  why  this  especial  case  has  been  chosen  as 
an  example  of  the  use  of  crystallisation  in  chemical 
industries. 

As  has  been  said  already,  bromine  was  discovered 
in  the  mother-liquor  of  certain  sea-salts.  In  some 
parts  of  the  countries  around  the  Mediterranean  the 
sea-water  is  allowed  to  flow,  in  the  summer  months, 
into  shallow  basins,  called  salt-gardens,  which  are 
separated  from  the  sea  after  they  have  been  filled  with 
sea-water.  The  water  in  these  basins  soon  evaporates, 
because  of  the  high  summer  temperature  of  these 
parts  ;  hence  the  sea-water  becomes  so  concentrated 
that  the  common  salt,  which  is  present  to  the  extent 
of  about  27  per  cent,  in  all  sea- water,  can  no  longer 
remain  in  solution,  but  must  crystallise  out.  This  salt 
is  sent  into  the  market.  The  liquid  that  remains  above 
the  salt,  which  is  called  the  mother-liquor  of  sea-salts, 
is  found  by  analysis  to  contain  fair  quantities  of 
bromine.  It  was  from  this  source  that  bromine  was 
first  obtained  in  the  year  1826. 

The  sea  long  remained  the  provider  of  bromine ; 
but  eventually  the  process  of  manufacturing  bromine 
passed  to  inland  countries,  especially  to  Stassfurt,  in 
the  neighbourhood  of  Magdeburg.  This  mining  centre 
is  the  only  place  at  present  known  to  have  a  practically 
inexhaustible  supply  of  potash  salts.  As  these  salts* 

*  We  shall  deal  with  these  compounds  in  some  detail  when 
we  come  to  the  metal  potassium. 


58       INTRODUCTION   TO   MODERN    CHEMISTRY. 

are  most  important  artificial  manures,  the  Stassfurt 
industries  supply  all  parts  of  the  world  with  this 
indispensable  aid  to  rational  agriculture.  The  Stass- 
furt mines  have  been  used  as  a  source  of  potash  salts 
since  the  sixties  of  the  nineteenth  century.  Before 
that  time,  potashes — that  is  potassium  carbonate — 
was  the  chief  source  of  the  potash  salts  required  for 
various  purposes.  But  potassium  carbonate  is  much 
too  expensive  to-day  to  be  used  for  making  artificial 
manure. 

The  accumulation  of  vast  deposits  of  potash  salts 
in  the  neighbourhood  of  Stassfurt  is  explained  if  we 
suppose  that  once  there  was  there  a  deep  arm  of  the 
sea  connected  with  the  main  ocean  only  by  a  shallow 
canal,  that  as  the  water  in  the  basin  evaporated  its 
place  was  supplied  by  water  flowing  in  from  the  sea, 
and  that  at  some  time  the  basin  was  cut  off  from  the 
main  ocean  by  the  silting  up  of  the  canal  or  by  some 
upheaval  of  the  earth.  As  the  water  of  what  was 
now  an  inland  sea  became  more  and  more  concentrated 
by  evaporation,  the  common  salt  must  have  crystallised 
out  gradually.  This  substance  is  found  in  largest  quan- 
tity in  the  lowest  parts  of  the  Stassfurt  deposits.  But, 
as  time  passed,  the  mother-liquor  above  the  separated 
salt  must  have  become  constantly  more  concentrated, 
until  at  last  it  dried  up,  and  so  left  behind  the  potash 
salts  it  contained  in  the  form  of  solid  masses.  Sea- 
water  contains  small  quantities  of  potash  salts  besides 
larger  quantities  of  sodium  chloride  (common  salt). 
Because  of  the  small  quantity  of  them,  these  potash 
salts  remain  in  the  mother-liquor  when  sea-salts  are 
separated  from  sea- water  in  the  way  already  described, 


PROPERTIES   OF    BROMINE.  59 

and  they  do  not  crystallise  out  until  that  mother-liquor 
is  concentrated  again.  The  proportion  of  common  salt 
to  potash  salts  in  the  Stassfurt  deposits  shows  that 
something  has  occurred  there  similar  to  what  we 
caused  to  take  place  when  we  separated  benzoic  acid 
by  crystallisation  from  a  little  salt  wherewith  we  had 
mixed  it.  In  that  process  one  of  the  substances  crys- 
tallised out  while  the  other  remained  in  the  mother- 
liquor. 

But  if  the  Stassfurt  potash  salts  are  derived  from 
the  sea,  if  they  have  been  separated  in  a  huge  natural 
sea-salt  factory,  they  must  contain  compounds  of 
bromine,  judging  from  what  we  know  of  the  composi- 
tion of  artificially  prepared  sea-salts.  And  they  do 
contain  bromine  compounds. 

The  crude  potash  salts  found  at  Stassfurt  must  be 
purified  for  the  purposes  of  agriculture  :  this  is  done 
by  crystallising  from  hot  water.  The  mother-liquors 
left  after  crystallising  out  the  potash  salts  contain  the 
bromine  compounds  from  which  the  element  bromine 
is  obtained.  So  great  is  the  demand  for  potash  salts 
that  there  are  sufficient  quantities  of  these  mother- 
liquors  at  Stassfurt  to  furnish  more  bromine  than  is 
required  by  the  whole  of  Europe.  Besides  the  chief 
factory,  there  are  a  few  salt-works  in  North  America 
from  the  mother-liquors  of  which  bromine  is  obtained. 

The  chemical  behaviour  of  bromine  towards  other 
bodies  is  exceedingly  like  that  of  chlorine,  only  every- 
thing is  slightly  weakened,  so  to  speak.  Chlorine  is 
a  yellow-green  gas ;  bromine  is  a  dark  red  liquid, 


60        INTRODUCTION    TO   MODERN    CHEMISTRY. 

which  can  be  gasified  very  easily,  as  it  boils  at  58°  C. 
[136°  F.].  Chlorine  reacts  with  almost  everything; 
bromine  reacts  similarly,  but  more  slowly.  Bromine 
is  only  slightly  less  dangerous  than  chlorine  in  its 
action  on  the  lungs  ;  and  so  on. 

Bromine  finds  its  chief  application  in  photography. 
Bromine  can  be  united  with  potassium  to  form  potassium 
bromide.  It  has  been  remarked  already  that  such 
compounds  as  this  are  called  salts.  This  word  has 
a  very  comprehensive  meaning :  we  shall  learn  more 
about  what  it  means  as  we  proceed.  ^  Potassium 
bromide  dissolves  in  water :  if  a  solution  of  silver 
nitrate  is  added  to  this  solution,  silver  bromide  separates 
at  once,  because  that  compound  is  quite  insoluble  in 
water,  while  potassium  nitrate  remains  in  solution 
(see  p.  50). 

Potassium  bromide  +  silver  nitrate  =  silver  bromide  +  potassium  nitrate. 
(Soluble  in  water)  (soluble  in  water)  (insoluble  in  water)  (soluble  in  water) 

Silver  bromide  for  photographic  purposes  is,  of 
course,  prepared  in  the  dark.  The  two  solutions  are 
mixed  in  a  room  into  which  light  is  not  admitted. 

Potassium  bromide,  and  also  the  two  very  similar 
salts  sodium  bromide  and  ammonium  bromide,  are 
used  as  drugs.  Small  quantities  of  these  salts  are 
sometimes  added  to  Seltzer  water  to  give  the  liquid 
a  soothing  action  on  the  nerves.  Bromine  is  also 
made  use  of  in  the  aniline-colours  industry  ;  but  the 
reactions  are  too  complicated  to  be  considered  here. 

The  total  consumption  of  bromine  is  not  very  great ; 
about  750,000  kilos,  [approximately  730  tons]  are  used 
annually  for  all  purposes. 


PREPARATION    OF   IODINE.  6 1 

IODINE. 

Iodine,  like  bromine,  is  found  in  sea-water ;  but  the 
quantity  of  iodine  is  so  small  that  the  direct  preparation 
of  this  element  from  sea-water,  or  even  from  the 
mother-liquors  thereof,  is  impracticable.  The  prepara- 
tion from  sea-water  is  made  possible,  however,  by  the 
following  facts.  Certain  sea-plants,  as  they  grow, 
withdraw  iodine  compounds  from  sea-water.  These 
plants  are  collected,  dried,  and  burnt,  in  order  to  obtain 
their  ashes  ;  and  these  ashes  contain  not  inconsiderable 
quantities  of  iodine  compounds.  It  was  from  the  ashes 
of  sea-plants  that  iodine  was  first  prepared  in  1811. 

The  ashes  of  sea-plants  were  the  sole  source  of 
iodine  until  the  middle  of  the  seventies  of  the  nine- 
teenth century,  when  a  new  and  more  abundant  source 
was  made  available.  We  have  already  learnt  some- 
thing about  Chili  saltpetre,  and  we  know  that  it 
crystallises  from  a  liquid  which  yields  a  mother-liquor 
after  the  removal  of  the  saltpetre.  That  mother-liquor, 
as  has  been  mentioned  before,  contains  a  little  iodine, 
the  extraction  of  which  has  been  conducted  since  the 
time  mentioned.  The  large  quantity  of  these  mother- 
liquors  which  is  available  makes  it  possible  for  Chili 
to  supply  more  iodine  than  is  demanded  by  the  whole 
world,  so  that,  as  in  the  case  of  bromine,  the  whole  of 
the  mother-liquors  existing  at  any  time  are  not  worked 
up.  When  we  are  dealing  with  manganese  we  shall 
learn  the  methods  whereby  the  mother-liquors  are 
manipulated. 

In  order  to  prepare  iodine  we  shall  proceed  just  as 


62         INTRODUCTION    TO    MODERN    CHEMISTRY. 


we  did  in  preparing   bromine,  and    we    shall    use    an 
apparatus  similar  to  that  we  used  then. 

This  time  hydriodic  acid  is  heated  with  manganese 
peroxide  in  the  retort  A  (fig.  25).  As  in  the  case  of 
hydrochloric  and  hydrobromic  acids,  so  here  the  oxygen 
of  the  peroxide  seizes  the  hydrogen  of  the  hydricdic 
acid  in  order  to  form  water,  with  the  result  that  the 
iodine  is  set  free.  Now,  iodine  is  a  solid  body  which 
does  not  melt  when  it  is  heated,  but  passes  directly 

into  the  state  of 
gas :  in  this  re- 
spect it  differs 
from  most  ordi- 
nary solid  bodies 
that  are  capable  of 
being  volatilised. 
When  such  bodies 
are  heated  they 
generally  melt  be- 
fore they  become 
gases ;  as  exam- 
ples we  recall  ice, 
water,  and  water-vapour.  Moreover,  when  iodine 
vapour  is  cooled  it  passes  from  the  state  of  gas  to 
that  of  a  solid:  there  seems  to  be  no  intermediate 
liquid  condition  for  this  substance.  Crystals  of  iodine, 
therefore,  separate  on  the  colder  parts  of  the  appa- 
ratus wherein  the  preparation  is  conducted — in  our 
figure  (25)  in  the  receiver  B.  One  does  not  say  of 
bodies  that  behave  like  iodine  "they  distil,"  but  "they 
sublime."  Iodine  belongs  to  the  sublirnable  bodies. 
To  get  it  perfectly  pure  it  is  sublimed  repeatedly. 


Fig.  25. — Preparation  of  iodine. 


PROPERTIES   OF  IODINE.  63 

Iodine  is  a  greyish  black  body,  having  the  specific 
gravity  4*95.  It  forms  a  violet  vapour  when  heated  : 
we  have  noticed  vapours  of  this  colour  filling  the  retort 
wherein  we  conducted  our  preparation  of  iodine.  The 
Greek  word  uoSrjs  signifies  violet-blue,  and  the  name 
iodine  given  to  the  element  has  reference  to  the  colour 
of  its  vapour,  or,  to  put  it  more  correctly,  to  the 
appearance  of  the  body  in  the  state  of  gas.  Iodine 
is  slightly  soluble  in  water,  but  it  dissolves  easily  in 
alcohol,  and  such  a  solution  forms  the  iodine  tincture 
of  the  Pharmacopeia  which  is  applied  externally  for 
several  purposes. 

Iodine  is  one  of  those  bodies  which  can  be  recognised 
in  very  minute  quantities ;  even  an  exceedingly  dilute 
solution  of  it  gives  a  deep  blue  colour  with  starch 
paste.  The  fact  has  been  established  quite  recently  that 
iodine  is  a  normal  constituent  of  the  human  body.  It  is 
found,  in  what  chemists  call  small  traces,  in  the  thyroid 
gland,  which  is  one  of  the  glands  of  the  neck.  The 
enlargement  of  this  gland  produces  goitre.  Since  iodine 
has  been  found  in  the  thyroid  gland  the  well-known 
beneficial  action  of  iodine  in  goitre  has  become  explicable. 

A  solution  of  iodine  is  much  too  corrosive  in  its 
action  to  allow  of  its  being  employed  internally. 
When,  therefore,  it  is  to  be  administered  internally,  it 
is  given  in  the  form  of  one  of  its  salts,  and  most 
commonly  in  the  form  of  potassium  iodide,  which  is 
easily  dissolved  by  water.  This  salt  has  absolutely  no 
corrosive  action — a  fact  which  cannot  be  at  all  strange 
to  us,  for  the  circumstances  are  quite  the  same  with 
common  salt  (sodium  chloride),  for  example.  We  know 


64       INTRODUCTION   TO   MODERN    CHEMISTRY. 

already  that  it  would  be  quite  impossible  to  introduce 
chlorine  regularly  into  the  human  system  ;  while,  on 
the  other  hand,  this  element  combined  with  sodium — 
that  is  to  say,  common  salt — is  altogether  indispensable. 

We  have  prepared  iodine  by  a  method  which  is 
exactly  comparable  with  those  whereby  chlorine  and 
bromine  are  produced.  If  we  add  a  solution  of  silver 
nitrate  to  a  solution  of  potassium  iodide,  we  obtain 
insoluble  silver  iodide  along  with  potassium  nitrate. 
This  reaction  is  similar  to  those  that  occur  with  chlorine 
and  bromine  compounds.  To  sum  up,  a  detailed 
examination,  carried  out  in  diverse  directions,  shows 
that  iodine  is  similar  in  every  respect  to  chlorine  and 
bromine.  It  is  also  used  for  purposes  like  those  for 
which  these  elements  are  employed.  The  yearly  con- 
sumption of  iodine  throughout  the  world  amounts  to 
about  600,000  kilograms  [approximately  600  tons]. 

FLUORINE. 

The  existence  of  fluorine  as  an  element  has  long 
been  established.  Compounds  of  it  have  been  known 
since  olden  days :  fluorspar,  for  instance,  which  is 
calcium  fluoride,  occurs  abundantly  in  nature.  The 
preparation  of  fluorine  in  the  free  state — that  is,  uncom- 
bined  with  any  other  element — dates,  however,  only  from 
1886.  Fluorine  is  not  only  the  most  active  member  of 
the  group  of  four  elements  we  are  now  considering — 
which  four  elements  are  called  the  group  of  the  halogens 
— but  it  is  the  most  active  of  all  the  elements.  An 
explanation  of  the  word  halogen  cannot  be  given  until 


PREPARATION  OF  FLUORINE.        65 

we  reach  a  later  part  of  this  book.  Because  of  its 
extraordinary  readiness  to  combine  with  other  sub- 
stances, fluorine  attacked  all  the  vessels  wherein 
attempts  were  made  to  prepare  it — and  these  attempts 
were  numberless ;  hence  it  could  not  be  obtained  un- 
combined  with  other  elements.  Methods  which  ought 
to  give  fluorine,  arguing  from  its  similarity  to  chlorine, 
bromine,  and  iodine — the  action  of  ^hydrofluoric  acid  on 
manganese  peroxide,  for  instance — did  not  enable  the 
element  to  be  obtained.  Instead  of  free  fluorine,  a 
compound  of  it  with  some  constituent  of  the  material  of 
the  vessel — with  a  constituent  of  glass,  for  instance — 
was  always  obtained.  At  last,  however,  the  element 
was  secured  by  passing  an  electric  current  through  its  / 
hydrogen  compound  (hydrofluoric  acid)  contained  in  a 
vessel  made  mainly  of  platinum,  at  a  low  temperature, 
which  exerts  a  moderating  influence  on  all  reactions, 
and  therefore  diminishes  the  energy  of  fluorine.  We 
have  already  learned,  from  our  experiment  with  water 
(p.  33),  that  the  electric  current  is  able  to  decompose 
chemical  compounds  into  their  elements.  In  this  case 
also,  when  the  conditions  were  suitable,  the  current 
retained  its  power,  and  it  became  possible  to  separate 
hydrofluoric  acid  into  its  two  constituents,  hydrogen 
gas  and  the  long-sought-for  fluorine. 

Fluorine  is  a  pale  yellow-green  gas,  resembling 
chlorine,  but  much  more  energetic  in  its  actions  than  that 
element.  For  example,  it  combines  energetically  with 
hydrogen,  even  in  the  dark,  when  it  is  mixe4  with,  that 
gas.  Chlorine  does  this  only  in  sunlight^ 


THE  HYDROGEN  COMPOUNDS  OF  CHLORINE, 
BROMINE,  IODINE,  AND  FLUORINE. 

Now  that  we  have  learned  something  of  the  four 
elements  of  the  halogen  group,  considered  by  themselves, 
the  best  way  of  proceeding  to  increase  our  chemical 
knowledge  is  to  consider  the  compounds  of  these 
elements  with  hydrogen,  wherewith  we  have  a  more 
intimate  acquaintance.  Each  of  the  four  elements 
forms  only  one  compound  with  hydrogen.  This  fact 
should  be  noted  carefully,  as  we  shall  find  it  to  be  of 
striking  importance  in  considering  the  whole  theoretical 
foundation  of  chemistry.  These  four  compounds — 
hydrogen  chloride,  hydrogen  bromide,  hydrogen  iodide, 
and  hydrogen  fluoride — are  all  gaseous  bodies,  although 
bromine  is  a  liquid  and  iodine  is  a  solid. 

HYDROCHLORIC  ACID  GAS. 

Hydrogen  chloride  is  one  of  those  gases  which  are 
very  soluble  in  water.  An  aqueous  solution  of  this 
compound  is  called  hydrochloric  acid  (sometimes 
muriatic  acid);  in  other  words,  commercial  hydro- 
chloric acid  is  water  wherein  hydrogen  chloride  gas 
js  dissolved, 

66 


PREPARATION    OF   HYDROCHLORIC  ACID.          67 

The  gas  is  obtained  by  warming  common  salt  with 
sulphuric  acid.  If  we  give  the  name  sodium  hydro- 
chloride  to  common  salt,  which  is  a  compound  of 
sodium  and  chlorine,  we  shall  perhaps  better  follow 
the  preparation  of  hydrochloric  acid  gas.  Sulphuric 
acid,  which  is  said  by  chemists  to  be  a  stronger  acid 
than  hydrochloric  acid,  drives  the  latter  acid  out  of 
the  sodium  hydrochloride,  and,  taking  the  place  of  the 
hydrochloric  acid,  forms  a  compound  with  the  sodium. 

Sodium  hydrochloride  +  sulphuric  acid  =  hydrochloric  acid 
gas  +  sodium  sulphate. 

We  shall  employ  the  apparatus  shown  in  fig.  26  for 
preparing  hydrochloric  acid  gas  and  a  solution  of  that 
gas  in  water.  The  apparatus  resembles  that  employed 
in  the  manufacture  of  the  acid.  Common  salt  and 
sulphuric  acid  are  placed  in  the  flask  A  ;  as  soon  as  this 
mixture  is  warmed,  a  regular  evolution  of  gas  begins, 
and  is  made. evident  by  the  bubbles  which  we  see 
passing  through  the  washing-flasks  D  and  E.  As  the 
apparatus  remains  quite  transparent  and  we  cannot 
see  the  gas  therein,  we  conclude  that  hydrochloric  acid, 
in  contradistinction  to  the  yellow-green  chlorine,  is  a 
colourless  gas.  If  we  open  the  stopcock  B,  so  that  the 
gas  streams  into  the  air  at  c,  we  notice  that  white  clouds 
form  immediately  at  the  place  where  the  gas  comes 
into  contact  with  the  air.  These  clouds  indicate  that 
the  hydrochloric  acid  gas  at  once  draws  to  itself  the 
moisture  of  the  air,  and  dissolves  therein  to  form  a 
solution  of  hydrochloric  acid,  the  minute  globules 
whereof  are  visible  as  a  cloud.  This  teaches  us  that 
hydrochloric  acid  gas  is  very  eager  to  draw  water  to 


68        INTRODUCTION   TO   MODERN   CHEMISTRY. 

itself  and  to  dissolve  therein.  If  we  now  close  the 
stopcock,  we  notice  that,  as  soon  as  all  the  air  is  driven 
out  of  the  apparatus  by  the  stream  of  gas,  the  gas 
coming  from  the  flask  wherein  the  reaction  is  proceed- 
ing is  completely  absorbed  by  the  water  in  the  first 
washing-flask  D.  A  considerable  time  is  required  for 


Fig.  26. — Preparation  of  hydrochloric  acid. 

the  complete  saturation  of  the  small  quantity  of  water 
in  D  by  the  hydrochloric  acid  gas,  for  the  gas  is  very 
soluble  in  water.  When  the  water  in  D  is  saturated,  the 
gas  passes  through  that  washing-flask  and  is  absorbed 
in  E. 

In  order  to  prevent  the  escape  of  any  hydrochloric 


PREPARATION   OF   HYDROCHLORIC  ACID. 


69 


acid  gas  into  the  room — for  the  horribly  penetrating 
smell  of  the  gas  would  soon  make  it  impossible  to 
remain  in  the  room — we  have  placed  a  tower  after 
the  washing-flasks.  This  glass  apparatus  represents 
the  very  large  towers,  built  of  brick  or  stone,  that 
are  used  in  chemical  works  where  hydrochloric  acid 
is  made.  In  such  works,  where  perhaps  100  kilos. 


Fig.  27. — Earthenware  jars  used  in  the  manufacture  ot  hydrochloric  acid. 

[say  200  lb.]  of  common  salt  are  decomposed  by 
sulphuric  acid,  in  a  properly  constructed  apparatus, 
about  thirty-six  earthenware  pots,  containing  water,  are 
placed  one  after  the  other,  for  the  purpose  of  absorbing 
the  acid  gas  (these  are  represented  in  our  apparatus  by 
the  two  washing-flasks)  ;  and  the  final  absorbing  tower 
is  more  than  twenty  metres  [say  sixty-five  feet]  high. 

Fig.  27  represents  two  of  these  earthenware  pots, 
sometimes  called  touries.     The  hydrochloric  acid  gas 


70       INTRODUCTION   TO   MODERN   CHEMISTRY. 

enters  at  A,  and  passes  through  the  water  in  the  pot 
into  another  pot,  and  so  on  to  the  tower.  A  stream 
of  water  is  kept  constantly  flowing  through  the  pots  in 
the  opposite  direction  to  that  wherein  the  gas  passes, 
for  the  purpose  of  absorbing  the  gas.  As  the  solution 
of  hydrochloric  acid  that  is  produced  is  heavier  than 
water,  the  constantly  flowing  stream  of  water  will 
drive  the  liquid  laden  with  hydrochloric  acid  from  the 
lower  part  of  one  pot  into  the  pot  next  nearer  the 
furnace.  The  illustration  shows  a  pipe  (D)  passing 
from  a  tubulus  in  the  side  of  each  pot  to  the  bottom 
of  the  next  pot.  The  overflow  from  one  pot  to  the  next 
must  pass  through  this  pipe.  A  completely  saturated 
aqueous  solution  of  hydrochloric  acid  flows  from  the 
last  pot:  this  solution  is  sold  as  hydrochloric  (or 
muriatic)  acid.  The  first  tower  contains  coke  which  is 
besprinkled  with  water,  as  was  done  on  the  small  scale 
in  the  glass  tower  of  our  apparatus,  into  which  water 
dropped  from  the  tap  (see  fig.  26).  The  very  large 
water-surface  which  is  exposed  by  the  wet  coke  insures 
that  the  last  traces  of  the  hydrochloric  acid  gas  are 
given  every  opportunity  to  dissolve  in  water,  and  so 
prevents  the  escape  into  the  air  of  any  of  the  acid  gas. 
This  method,  employed  in  factories  for  the  complete 
absorption  of  the  hydrochloric  acid  gas,  is  practised  also 
in  our  small  apparatus,  from  which  no  acid  vapours 
escape  into  the  room,  as  is  made  evident  by  the  fact 
that  we  do  not  see  any  formation  of  clouds  round  the 
upper  opening  of  the  tower. 

The  hydrochloric  acid  which  we  obtain  in  our  wash- 
ing-flasks  is   colourless,    while   the   commercial    acid, 


PROPERTIES   OF   HYDROCHLORIC   ACID.  Jl 

wherewith  our  acid  should  be  identical  unless  especial 
means  are  taken  to  make  it  very  pure,  is  yellowish  as 
it  flows  from  the  pots.  The  reason  for  this  difference 
in  the  appearances  of  the  laboratory-product  and  the 
acid  made  on  the  large  scale  is  that  the  commercial 
acid  has  a  little  iron  dissolved  in  it,  which  iron  comes 
from  some  of  the  many  pieces  of  apparatus  made 
of  iron  or  containing  iron  that  are  used  in  the 
chemical  works.  As  our  apparatus  is  made  wholly  of 
glass,  the  presence  of  iron  in  our  acid  is,  of  course, 
avoided.  If  we  put  a  few  rusty  nails  into  the  colourless 
acid  we  have  made,  that  acid  will  soon  show  the  same 
yellow  colour  as  the  commercial  product. 

Liquid  hydrochloric  acid — that  is,  a  solution  of 
hydrochloric  acid  gas  in  water — fumes  in  the  air  just 
as  the  gas  does ;  in  other  words,  if  we  open  a  bottle 
containing  the  liquid  acid,  we  see  fumes  rising  from  the 
mouth  of  the  bottle.  The  reason  of  this  is  that  some 
hydrochloric  acid  gas  escapes  from  the  solution  and 
forms  clouds  in  the  air,  in  the  way  explained  already. 
It  is,  of  course,  only  the  very  concentrated  acid  that 
fumes  in  the  air — the  acid,  that  is,  which  is  so  con- 
centrated that  the  water  can  scarcely  retain  in  solution 
all  the  acid  gas  that  has  been  dissolved  in  it.  The 
acid  which  is  produced  by  the  manufacturer  is  generally 
made  as  concentrated  as  possible,  in  order  to  bring 
the  cost  of  transport  to  a  minimum.  If  the  "fuming 
acid"  is  diluted  with  water,  it  loses  the  property  of 
fuming  in  the  air,  for  there  is  now  enough  water  to 
keep  the  whole  of  the  hydrochloric  acid  gas  in  solution 
easily,  so  that  none  escapes  from  the  liquid.  Strong — 
that  is,  concentrated — hydrochloric  acid  contains  rather 


72       INTRODUCTION    TO   MODERN   CHEMISTRY. 

more  than  40  per  cent,  of  hydrochloric  acid  gas — that 
is,  more  than  400  grams  per  litre  [about  28,000  grains 
per  gallon]. 

Aqueous  hydrochloric  acid  has  already  been  used 
by  us  in  the  preparation  of  chlorine ;  we  oxidised  the 
hydrogen  of  the  acid  to  water,  and  the  chlorine  was 
set  free.  We  have  made  especial  use  of  manganese 
peroxide  as  a  means  of  effecting  oxidations.  This  com- 
pound is,  of  course,  not  the  only  substance  available 
for  the  purpose,  for  almost  any  compound  which  is 
very  rich  in  oxygen  will  enable  us  to  get  chlorine  from 
hydrochloric  acid.  Manganese  peroxide  is,  however, 
the  cheapest  material  that  can  be  used  for  the  purpose. 
As  we  know  that  hydrochloric  acid  is  obtained  from 
common  salt  by  a  very  simple  process,  we  can  easily 
see  that  the  chlorine  which  is  got  from  the  acid — and 
hence  such  other  chlorine  preparations  as  chloride 
of  lime  (bleaching  powder) — ought  not  to  be  very 
expensive. 

ACIDS,  BASES,  AND  SALTS. 

Although  we  have  only  now  prepared  one  acid — 
namely,  hydrochloric  acid — we  could  not  avoid  the  use 
of  the  term  acid  in  the  earlier  parts  of  this  book. 
The  word  acid  originally  designated  substances  which 
have  a  sour  taste  or  a  sour  smell,  such  as  vinegar, 
hydrochloric  acid,  and  sulphuric  acid.  It  is  evident 
that  chemistry  could  not  remain  satisfied  with  a  refer- 
ence to  this  purely  external  property,  especially  as 
when  acids  are  very  greatly  diluted  by  the  addition 
of  much  water  the  taste  and  the  smell  must  generally 


ACIDS,   BASES,   AND   SALTS.  73 

vanish,  although  the  liquids  still  contain  acids  in  the 
chemical  meaning  of  the  word.  Now  it  was  found,  a 
long  time  ago,  that  acids  change  the  colours  of  many 
vegetable  colouring  matters  even  when  the  acids  are 
greatly  diluted.  The  blue  colouring  matter  called  litmus, 
which  is  found  in  many  species  .of  lichens  and  is  a 
substance  easily  obtained,  is  one  of  those  bodies  which 
are  readily  changed  by  acids.  This  substance  is  very 
soluble  in  water  :  its  blue  aqueous  solution  is  turned 
red  by  the  smallest  trace  of  any  acid.  This  very 
convenient  means  of  detecting  acids  is  far  superior  to 
the  tests  of  taste  and  smell ;  but  as  this  also  is  purely 
an  external  and  accidental  means,  it  cannot  be  regarded 
as  sufficient  in  scientific  work.  There  are  liquids 
which  cause  litmus  solution  that  has  been  reddened 
by  acids  to  turn  blue  again.  These  liquids  behave 
towards  such  a  vegetable  colouring  matter  as  litmus 
in  a  way  directly  the  opposite  of  that  of  acids ;  they 
annul,  or,  it  may  be  said,  they  neutralise,  the  effects 
produced  by  acids.  One  says  of  such  liquids  which 
turn  reddened  litmus  blue  that  they  have  a  basic,  or 
alkaline,  reaction.  These  two  expressions,  base  and 
alkali,  are  used  to-day  as  synonymous ;  *  we  shall 
employ  them  both  without  any  difference  of  meaning. 
The  word  base  is  derived  from  the  Latin  basis]  the 
word  alkali  is  of  Arabic  origin.  Basic,  or  alkaline, 
liquids  do  not  taste  sour,  but  rather  saline.  They 
are  not  less  corrosive  than  the  acids ;  the  expres- 
sion caustic  ley,  for  instance,  implies  the  possession 
of  corrosive  power.  (We  shall  soon  consider  this 

*  In  English  books  on  chemistry  the  term  base  is  generally 
used  with  a  wider  signification  than  the  word  alkali.    [TR.] 


74       INTRODUCTION   TO   MODERN    CHEMISTRY. 

substance  more  fully.)  If  the  reactions  that  occur 
when  bases  are  neutralised  by  acids  are  followed  more 
narrowly,  we  find  that  the  acids  combine  with  the 
bases  to  form  a  special  class  of  compounds,  and  that 
the  opposing  functions  both  of  the  acids  and  the  bases 
are  balanced,  so  thfct  the  products  of  these  actions 
have,  generally,  no  action  on  the  colour  of  litmus. 

That  which  is  formed  by  the  reaction  of  an  acid  with 
a  base  is  called  a  salt.  A  great  number  of  acids  and 
bases  is  known  ;  hence  a  great  number  of  salts  is 
produced  by  the  reactions  of  these  acids  and  bases. 
In  ordinary  language  the  word  salt  is  applied  to  one 
substance  only — common  salt.  No  mention  has  been 
made  in  the  foregoing  statements  of  the  external 
characteristics  of  acids,  bases,  and  salts.  It  is  not  at 
all  necessary  that  acids  and  bases  should  be  liquids, 
as  one  might  be  apt  to  suppose  when  beginning  the 
study  of  these  bodies,  nor  that  al^^them  should  act 
on  vegetable  colours  ;  the  esserM  Bint  is  this — what 
combines  with  an  acid  to  form  aj^Jis  a  base,  or,  con- 
versely, what  combines  with  a  base  to  form  a  salt  is  an  acid. 


Hydrochloric  acid  is  not  a  liquid,  but  a  solution 
of  a  gas  in  water ;  on  the  other  hand,  sulphuric  acid 
and  acetic  acid  (the  acid  of  vinegar)  are  liquids.  The 
acids  hitherto  spoken  of,  which  are  such  good  repre- 
sentatives of  the  general  type  of  acid,  colour  litmus 
red  ;  but  sand  does  not  do  so.  Nevertheless,  sand  is 
an  acid  (chemists  call  it  silicic  acid)  ;  it  combines  with 
bases — with  potash,  for  instance,  it  forms  a  salt  called 
potassium  silicate. 


ACIDS,  BASES,  AND  SALTS.          75 

The  bases,  or  alkalis,  also  may  be  solid,  liquid, 
or  gaseous  bodies.  The  bases  caustic  potash  and 
caustic  soda  are  solids.  Aniline  is  a  liquid  base  : 
although  it  does  not  colour  reddened  litmus  blue, 
it  is  manifestly  a  base,  for  it  combines  with  sul- 
phuric acid  to  form  a  salt — aniline  sulphate — and 
with  hydrochloric  acid  to  form  a  salt — aniline  hydro- 
chloride.  Ammonia  is  a  gaseous  base,  which  we  shall 
consider  hereafter  pretty  fully,  as  it  is  of  great  general 
interest.  It  is  true  that  common  salt  is  soluble  in 
water ;  but  solubility  or  insolubility  in  water  has  no 
importance  in  the  chemical  conception  of  salt :  as  a 
matter  of  fact,  a  great  many  salts  are  insoluble  in 
water — for  instance,  lead  carbonate  and  calcium  silicate. 
But  a  further  treatment  of  the  conception  of  salt  must 
be  deferred  until  a  later  period  of  this  book. 

Which  are  the  elements  compounds  whereof  are 
acids,  and  of  what  elements  are  bases  composed  ? 
The  following  is  a  broad  and  general  answer  to  this 
question.  When  we  come  to  deal  with  individual 
elements  we  shall  have  many  illustrations  of  what  is 
said  here. 

The  oxides  of  the  non-metallic  elements  react  with 
water  to  produce  acids — we  speak  of  sulphuric  acid, 
phosphoric  acid,  and  so  on.  The  oxides  of  the  metals 
give  the  bases — as  lead  oxide,  copper  oxide,  and  the 
like.  A  complete  exception  to  this  statement,  which 
holds  good  only  broadly  and  generally,  is  furnished  by 
the  group  of  elements  chlorine,  bromine,  iodine,  and 
fluorine,  for  the  compounds  of  these  with  hydrogen 
alone  are  acids. 


76       INTRODUCTION   TO   MODERN   CHEMISTRY. 


HYDROBROMIC,  HYDRIODIC,  AND  HYDROFLUORIC 
ACID. 

Now  that  we  know  something  of  acids,  bases,  and 
salts,  and  have  considered  hydrochloric  acid  in  some 
detail,  we  must  review  the  three  acids  hydrobromic, 
hydriodic,  and  hydrofluoric. 

Hydrobromic  and  hydriodic  acids  resemble  hydro- 
chloric acid  in  every  way.  They  are  colourless  gases, 
very  soluble  in  water ;  hence,  like  hydrochloric  acid, 
they  are  kept  in  aqueous  solutions,  that  being  the  most 
convenient  way  of  storing  them.  With  regard  to  hydro- 
fluoric acid,  which  is  a  solution  in  water  of  hydrofluoric 
acid  gas,  it  is  especially  to  be  noted  that,  in  contradis- 
tinction to  the  other  three  acids,  it  must  not  be  kept  in 
glass  vessels,  because,  like  fluorine  itself,  it  attacks 
glass.  The  preparation  of  this  acid  is  chemically  similar 
to  that  of  hydrochloric  acid  ;  but  leaden  vessels  must 
be  used,  because  these  are  not  acted  on  by  the  acid, 
although  they  are  corroded  by  fluorine.  This  acid  is 
kept  in  flasks  made  of  caoutchouc,  which  are  much 
lighter  than  leaden  vessels,  because  experiments  have 
shown  that  the  acid  does  not  act  on  this  material.  The 
acid  is  known  commercially  as  fluoric  acid,  because  for 
the  last  two  hundred  years  it  has  been  prepared  from 
fluorspar  (calcium  fluoride),  for  the  purpose  of  etching 
glass,  by  heating  that  substance  with  sulphuric  acid. 
In  making  hydrochloric  acid  we  started  with  sodium 
chloride,  a  substance  found  native  in  abundance. 
Sodium  fluoride  is  not  tfound  in  nature,  but  calcium 


ETCHING  GLASS.  77 

fluoride  is  a  mineral ;  in  this  case,  therefore,  we  use 
calcium  fluoride  : — 

Calcium  fluoride  4-  sulphuric  acid  ==  hydrofluoric  acid  + 
calcium  sulphate. 

Hydrofluoric  acid  is  chiefly  used  for  etching  glass — 
that  is,  for  fixing  lines  or  writing  on  glass.  The  glass 
is  covered  with  wax  or  paraffin  by  allowing  one  of 
these  substances  to  melt  on  the  warm  glass  and  then 
running  it  over  the  surface  ;  after  cooling,  the  desired 
outline  is  cut  through  the  covering  material,  and  hydro- 
fluoric acid  is  poured  on.  The  exposed  parts  of  the  glass 
are  etched  in  a  few  minutes.  The  acid  is  then  washed 
away,  and  the  covering  is  removed  by  melting  it, 
whereupon  the  etching  is  visible.  The  applicability  of 
wax  or  paraffin  as  a  material  for  use  in  etching  with 
hydrofluoric  acid  has  been  established  by  purely 
practical  trials,  just  as  caoutchouc  flasks  have  been 
found  to  be  suitable  for  storing  the  acid. 


ATOMS   AND   THEIR   WEIGHTS. 

THE  method  we  have  used  in  advancing  our  knowledge 
of  chemistry  cannot  be  called  unscientific.  Neverthe- 
less, we  have  proceeded  hitherto  rather  after  the  manner 
of  the  descriptive  sciences,  such  as  botany  or  miner- 
alogy ;  but  chemistry  is  an  abstract  and  theoretical* 
natural  science.  We  have  contented  ourselves,  on  the 
whole,  with  discovering  the  external  properties  of  the 
bodies  we  have  been  considering,  and  how  their  actions 
on  one  another  manifest  themselves.  Hitherto  we 
have  paid  no  attention  to  the  quantities  of  the  sub- 
stances required  in  the  different  experiments.  But  as 
long  as  one  does  not  take  into  account  the  relations 
between  the  weights  of  the  substances  used  in  chemical 
investigations,  it  is  impossible  to  speak  of  making  any 
genuine  scientific  advance  in  chemistry.  The  serious 
use  of  the  balance  in  chemistry  dates  from  the  second 
half  of  the  eighteenth  century.  Before  that  time, 
speaking  broadly,  no  advance  was  made  beyond 
alchemy,  which  we  name  to-day  with  a  shrug  of 
the  shoulders. 

The  full  consideration  of  weight-relations  in  investi- 

*  The  expression  in  the  original  is  eine  spekulative  Naturwis- 
semchaft.    [TR.] 

78 


ATOMIC  WEIGHTS.  79 

gations  in  the  domain  of  chemistry  has  gradually  led 
to  those  great  results  which  have  insured  for  chemistry 
so  prominent  a  place  among  the  special  branches  of 
the  whole  body  of  natural  science.  We  must,  there- 
fore, as  we  proceed,  always  take  these  relations  into 
account. 

In  our  experiments  we  obtained  regular  streams 
of  chlorine,  hydrochloric  acid,  and  the  like  with  the 
greatest  ease.  But  if  anyone  were  to  try  to  prepare 
such  chemical  compounds  with  no  more  knowledge 
of  the  reactions  than  he  may  have  acquired  from 
what  has  been  said  hitherto  in  this  book,  he  would 
find  in  most  cases,  little  to  his  delight,  that  matters 
are  not  so  simple  as  he  thought.  Either  he  would 
obtain,  in  all  probability,  such  violent  streams  of 
gas  that  he  would  hardly  know  how  to  guard  him- 
self against  them,  or  the  opposite  would  happen,  and 
the  production  of  gas  would  take  place  in  such  a 
feeble  way  that  he  would  not  be  able  to  do  anything 
with  it. 

Why  is  it,  then,  that  the  man  who  has  had  experience 
in  these  matters  is  able  to  regulate  such  processes  with 
ease  and  have  them  completely  under  his  control  ? 
The  reason  has  been  alluded  to  already;  it  is  that  in 
the  examinations  we  have  made  hitherto  of  reactions 
between  substances,  wherever  it  was  necessary,  the 
proper  relative  weights  of  the  reacting  bodies  have  been 
used.  The  substances  have  not  been  mixed  in  indefinite 
quantities.  But  how  is  it  possible  to  determine  be- 
forehand what  are  the  proper  quantities  by  weight  of 
any  two  chemical  compounds  which  should  be  used 
in  a  reaction  between  these  bodies  ?  What  are  the 


80       INTRODUCTION    TO   MODERN    CHEMISTRY. 

connections  between  two  such  different  substances  as 
common  salt  and  sulphuric  acid;  from  which  we  pro- 
duced hydrochloric  acid  gas,  which  enable  us  to 
calculate  the  proportions  by  weight  wherein  they 
must  be  mixed  ?  We  shall  try  to  express  ourselves 
clearly  on  this  point.  There  is  no  special  difficulty. 
But  these  fundamental  conceptions,  whereon  the 
whole  structure  of  scientific  chemistry  rests,  cannot 
be  mastered  in  a  hurry ;  they  must  be  thought  about 
quietly. 

We  have  already  become  acquainted  with  hydro- 
chloric acid  gas,  and  we  know  that  it  consists  of 
hydrogen  gas  and  chlorine  gas.  We  also  know  that 
hydrobromic  acid  gas  and  hydriodic  acid  gas  consist 
of  bromine  and  hydrogen,  and  iodine  and  hydrogen, 
respectively.  We  need  not  take  hydrofluoric  acid  gas 
into  consideration,  as  the  three  gases  we  have  mentioned 
will  suffice  for  our  purpose. 

If  we  determine,  by  the  help  of  a  balance  (and  the 
determinations  are  not  overpoweringly  difficult),  how 
much  chlorine,  how  much  bromine,  and  how  much 
iodine  is  combined,  in  one  or  other  of  the  three  com- 
pounds, with  one  part  by  weight  of  hydrogen — that  is, 
with  one  part  by  weight  of  the  specifically  lightest  of 
all  substances — we  find  that  with  unit  weight  of  hydro- 
gen there  is  combined  35*5  parts  by  weight  of  chlorine 
in  hydrochloric  acid,  80  parts  by  weight  of  bromine 
in  hydrobromic  acid,  and  127  parts  by  weight  of 
iodine  in  hydriodic  acid.  These  numbers,  which  ex- 
press parts  by  weight  of  the  three  elements — namely,  35*5 
for  chlorine,  80  for  bromine,  and  1 27  for  iodine — are 


ATOMIC  WEIGHTS.  8 1 

maintained  in  a  very  remarkable  way  in  the  compounds 
of  the  three  elements  with  other  elements,  for  instance, 
with  silver,  sodium,  etc. 

For  example,  we  know  that  common  salt  is  sodium 
chloride.  If  we  determine,  with  the  help  of  the  balance, 
how  many  parts  by  weight  of  sodium  are  combined  in 
this  compound  with  35-5  parts  of  chlorine — that  is, 
with  that  quantity  of  chlorine  which  combines  with  one 
part  by  weight  of  hydrogen — we  find  that  23  parts  by 
weight  of  sodium  are  so  combined.  If  we  now  find 
how  much  bromine  combines  with  23  parts  by  weight 
of  sodium  to  form  sodium  bromide,  the  result  is  80 
parts  of  bromine,  which  is  the  weight-number  we  have 
determined  for  bromine  from  the  composition  of  hydro- 
bromic  acid  gas.  Moreover,  we  find  that  23  parts  by 
weight  of  sodium  combine  with  127  parts  by  weight 
of  iodine  to  form  sodium  iodide;  but  127  is  the 
weight- number  determined  for  iodine  from  the  com- 
position of  hydriodic  acid  gas. 

To  go  farther.  If  we  throw  23  parts  by  weight 
of  sodium  into  water,  we  discover  that  this  quantity 
suffices  to  set  free  exactly  one  part  by  weight  of  hydro- 
gen (see  the  experiment  described  on  p.  29).  To 
put  the  matter  in  a  few  words,  23  parts  by  weight  of 
sodium  combine  with  exactly  the  same  quantities  by 
weight  of  chlorine,  bromine,  and  iodine  as  combine 
with  one  part  by  weight  of  hydrogen ;  and  23  parts  by 
weight  of  sodium  set  free  from  water  exactly  one  part 
by  weight  of  hydrogen. 

An  examination  of  the  quantities  by  weight  wherein 
elements  combine,  such  as  that  we  have  now  conducted 

6 


82       INTRODUCTION   TO   MODERN   CHEMISTRY. 

for  five  elements,  can,  of  course,  be  carried  out  for 
all  the  elements.  This  has  been  done.  The  result  is 
that  all  the  elements  combine  with  one  another  in 
fixed  proportions  by  weight,  which  never  change,  or 
in  moderately  large  simple  multiples  of  these  propor- 
tions, so  that  the  weights  of  the  elements  which 
combine  to  form  chemical  compounds  always  bear  the 
same  proportion  to  one  another.  Taking  chlorine,  for 
example,  we  always  find  that  35*5  parts  by  weight,  or 
a  simple  multiple  of  this  quantity,  of  chlorine  enters 
into  combination,  referred  to  one  part  by  weight  of 
hydrogen;  in  hydrochloric  acid,  35*5  parts  by  weight 
of  chlorine  are  combined  with  one  part  by  weight  of 
hydrogen  ;  in  compounds  which  contain  other  elements 
besides  these  two,  we  find  2  x  35'5,  or  3  x  35*5,  etc., 
parts  by  weight  of  chlorine.  It  may  almost  be  said 
that  a  certain  weight-number  adheres  to  each  element. 
What  has  been  said  is  absolutely  certain ;  there  is 
nothing  theoretical  about  it;  it  is  not  arrived  at  by 
speculative  thinking,  but  all  the  weight-numbers  are 
determined  by  the  use  of  the  balance,  which  eliminates 
all  errors. 

But  we  must  now  ask  whether  an  explanation  can 
be  found  of  these  facts,  which  have  been  established 
by  applying  the  balance  to  determine  the  proportions 
by  weight  wherein  the  elements  combine  with  one 
another.  As  the  invariability,  the  perpetual  con- 
stancy, of  the  proportions  by  weight  wherein  the 
elements  enter  into  their  compounds,  and  wherein 
we  find  them  in  their  compounds,  holds  good  for  all 
elements,  and  as  all  the  elements  taken  together 


ATOMIC   WEIGHTS.  83 

represent  the  totality  of  matter,  it  follows  immedi. 
ately  from  these  facts  that  what  we  have  found  to 
hold  good  must  depend  on  some  general  property 
of  matter. 

To  arrive  at  an  explanation  of  this  behaviour  of 
matter,  let  us  pursue  the  following  train  of  thought. 
We  hold  a  solid  bar  of  metal — say  a  bar  of  steel — in 
our  hand.  Now,  this  bar  will  either  altogether  and 
completely  fill  *the  place — or,  as  we  may  equally  well 
say,  the  space — wherein  it  is,  or  it  will  not  absolutely  fill 
that  space.  No  other  hypothesis  is  feasible.  Evidently 
no  chemical  knowledge  is  required  to  arrive  at  this 
conclusion ;  it  belongs  to  the  domain  of  philosophy, 
and  the  ancient  Greek  philosophers  busied  themselves 
with  conceptions  .about  the  filling  of  space  by  matter. 
The  careful  consideration  of  the  supposition  that 
matter  entirely  fills  space  soon  leads  to  conclusions 
which  no  branch  of  natural  science,  and  no  inquiry 
that  rests  as  much  as  possible  on  experiments  and 
facts  perceptible  by  our  senses,  can  make  anything  of. 
(It  is  true  that  certain  philosophers,  of  late  years,  have 
constructed  a  world  on  the  foundation  of  the  absolute 
filling  of  space  by  matter;  but  they  do  not  consider 
it  necessary  to  rest  their  conceptions  on  experimental 
evidence,  and  with  words  one  may  dispute  most 
excellently.)  On  the  other  hand,  everything  that  has 
been  observed  by  investigators  of  nature  can  be  pre- 
sented clearly  if  we  think  of  matter  as  not  filling 
space  entirely,  but  as  consisting  of  exceedingly  minute 
particles,  which  are  very  near  one  another,  but  not  so 
near  as  to  prevent  the  possibility  of  there  being  any 
interstices  between  them.  These  particles,  which  we 


84       INTRODUCTION    TO   MODERN    CHEMISTRY. 

may  think  of  as  spherical,  are  regarded  as  so  small 
that  they  are  not  further  divisible.  A  special  term  is 
applied  to  these  smallest  particles  supposed  by  this 
view  to  exist— namely,  the  term  atom,  a  word  which 
was  used  by  the  ancient  Greek  philosophers.  Trans- 
lated literally,  the  word  means  not  capable  of  being 
cut :  we  generally  render  it  by  indivisible.  We 
must  also  assume  that  the  smallest  particles  of  any 
particular  element  are  equally  large  'and  of  equal 
weights — for  instance,  that  all  the  atoms  of  hydrogen 
are  of  equal  size  and  equal  weight,  all  the  atoms 
of  chlorine  are  of  equal  size  and  equal  weight,  and 
so  on. 

To  recapitulate.  The  elements  are  composed  of 
atoms,  the  atoms  of  each  element  have  all  the  same 
weight,  and  the  weight  of  the  atoms  of  one  element 
differ  from  the  weights  of  the  atoms  of  the  other 
elements.  With  the  help  of  this  simple  conception 
it  is  easily  possible  to  determine  the  weights  of  the 
atoms  of  the  elements,  by  making  use  of  those 
numbers  which  are  determined  by  the  balance  and 
express  the  weights  of  the  elements  that  mutually 
combine.  It  is,  of  course,  impossible  to  make  direct 
weighings  of  atoms  ;  for  they,  the  smallest  parts 
of  matter  that  we  can  conceive,  are  naturally  so 
minute  that  no  human  eye  can  see  them  even  when 
aided  by  the  strongest  microscope,  no  finest  machinery 
can  handle  them,  no  most  Delicate  balance  can  weigh 
them.  But  their  weights  relatively  one  to  another 
— for  instance,  how  many  times  an  atom  of  chlorine 
is  heavier  than  an  atom  of  hydrogen — can  be 


ATOMIC   WEIGHTS.  85 

established  without  very  much  trouble  in  the  follow- 
ing manner. 

Notwithstanding  many  attempts  to  prepare  other 
compounds,  only  one  compound  of  chlorine  with  hydro- 
gen, one  of  bromine  with  hydrogen,  and  one  of  iodine 
with  hydrogen,  is  known.  These  three  compounds  are 
hydrochloric  acid,  hydrobromic  acid,  and  hydriodic  acid. 
As  only  one  compound  of  each  of  these  three  elements 
with  hydrogen  has  been  prepared,  despite  the  great 
trouble  that  has  been  taken  to  form  others,  one  is  really 
driven  to  adopt  the  hypothesis  that,  if  the  elements  are 
formed  of  atoms,  then  a  single  atom  of  chlorine,  a  single 
atom  of  bromine,  and  a  single  atom  of  iodine  is  com- 
bined with  a  single  atom  of  hydrogen  in  these  three 
compounds,  respectively.  Whatever  other  hypothesis 
may  be  made,  none  is  so  simple  as  this,  which  suggests 
itself  so  readily  and  is  not  contradicted  by  any  known 
facts.  If  this  supposition  is  accepted — and  it  com- 
mends itself  to  our  perceptions  and  satisfies  our  mode 
of  thought  more  than  any  other — then  we  at  once 
arrive  at  a  knowledge  of  the  weights  of  these  atoms, 
by  the  following  process.  We  know  that  35^5  parts  by 
weight  of  chlorine  are  combined  withfone  part  by  weight 
of  hydrogen  in  hydrochloric  acid :  according  to  our 
hypothesis,  this  compound  is  formed  of  one  atom  of 
hydrogen  and  one  atom  of  chlorine ;  hence  an  atom 
of  chlorine  is  35-5  times  heavier  than  an  atom  of 
hydrogen  ;  for,  as  the  total  quantity  of  chlorine  in 
hydrochloric  acid  is  35-5  times  heavier  than  the  total 
quantity  of  hydrogen,  the  ratio  of  the  weight  of  the 
smallest  quantity  of  chlorine  to  that  of  the  smallest 


86       INTRODUCTION   TO   MODERN   CHEMISTRY. 

quantity  of  hydrogen  in  that  compound  is,  of  course, 
also  as  35-5  to  I  ;  and  these  smallest  quantities  are  one 
atom  of  chlorine  arid  one  atom  of  hydrogen  respec- 
tively. If  we  carry  over  this  way  of  considering  the 
facts  to  hydrobromic  and  hydriodic  acids,  we  arrive  at 
the  conclusion  that  an  atom  of  bromine  is  80  times 
heavier,  and  an  atom  of  iodine  is  127  times  heavier, 
than  an  atom  of  hydrogen.  Nothing  is  left  out  of 
account  in  this  determination  of  atomic  weights ;  the 
weights  of  the  atoms  of  these  elements  are  determined 
completely. 

But  it  may  be  asked  why  we  are  entitled  to  take  the 
atomic  weight  of  hydrogen  as  unity.  We  do  that 
without  hesitation;  for  all  weights  must  be  finally 
referred  to  a  unit  weight,  which  must  be  accepted  once 
for  all.  This  is  done  in  the  ordinary  system  of  weights 
wherein  quantities  are  reckoned  in  grams.  For  in- 
stance, when  we  say  that  a  certain  vessel,  with  its 
contents,  weighs  735  grams,  what  we  assert  is  that 
the  weight  of  the  thing  is  735  times  greater  than  one 
gram.  It  has  been  agreed,  once  for  all,  that  the  weight 
of  one  cubic  centimetre  of  water  shall  be  called  a  gram, 
and  shall  be  the  unit  of  our  system  of  weights  where- 
with all  other  weights  shall  be  compared."  Exactly  in 
the  same  way,  in  chemistry  it  is  agreed  that  the  weight 

*  In  our  preposterous  English  weights  and  measures  there  is 
no  such  simple  relation  between  the  units  of  weight  and  volume 
as  exists  in  the  metric  system.  The  very  simple  relations 
between  the  units  of  weight,  volume,  and  length  of  the  metric 
system  constitute  the  great  advantage  of  that  system  over  all 
others.  £TR.] 


ATOMIC  WEIGHTS.  87 

of  an  atom  of  hydrogen  is  to  be  taken  as  the  unit  in 
terms  whereof  the  weights  of  all  other  atoms  shall  be 
stated. 

The  foregoing  explanation  will  enable  us  easily  to 
understand  the  proper  significations  of  the  abbreviated 
forms  of  the  names  of  the  elements,  and  the  chemical 
formulae  which  are  constructed  by  using  these  symbols. 
The  abbreviations  denote  the  elements  themselves,  as 
we  know  already  ;  but  they  also  signify  much  more  : 
each  signifies  one  atom  of  the  element  in  question.  Cl 
is  not  only  the  symbol  for  chlorine,  but  Cl  signifies  one 
atom  of  chlorine,  Fe  signifies  one  atom  of  iron,  and  so 
on.  As  the  atoms  represent  definite  weights,  so  do 
these  abbreviations  indicate  that  such  or  such  a  quan- 
tity by  weight  of  this  or  that  element  is  present  in  the 
compound  under  consideration.  The  formula  HC1 
signifies  not  only  hydrochloric  acid,  but  it  tells  us 
that  hydrochloric  acid  is  formed  by  the  union  of 
one  atom  of  hydrogen  with  one  atom  of  chlorine, 
and  also  that  this  compound  is  composed  of  one  part 
by  weight  of  hydrogen  and  35-5  parts  by  weight  of 
chlorine. 

Before  proceeding  to  make  use  of  the  knowledge  we 
have  now  gained,  we  shall  repeat  the  table  of  the 
elements  (given  on  p.  22),  adding,  after  the  abbre- 
viated form  of  the  name  of  each  element,  the  weight 
of  the  atom  of  that  element.  To  repeat  once  more  : 
these  weight-numbers  tell  us  how  many  times 
heavier  an  atom  of  each  element  is  than  an  atom  of 
hydrogen. 


88        INTRODUCTION   TO   MODERN    CHEMISTRY. 


LIST  OF  THE  ELEMENTS, 

AND   THE  ABBREVIATED   METHOD   OF  WRITING   THEIR   NAMES. 


Name  of  Element. 

Shortened 
form  of 
Name. 

Atomic 
weight. 

Name  of  Element. 

Shortened 
form  of 
Name. 

Atomic 
weight. 

Aluminium    . 

Al 

27-I 

Neodymium 

Nd 

144 

Antimony  (stibium) 

Sb 

120 

Neon    . 

Ne 

Argon  . 

A 

40 

Nickel. 

Ni 

587 

Arsenic         .         . 

As 

75 

Niobium 

Nb 

94 

Barium 

Ba 

137-4 

Nitrogen 

N 

14 

Beryllium     . 

Be 

9-1 

Osmium 

Os 

191 

Bismuth 

Bi 

208-5 

Oxygen 

O 

16 

Boron  . 

B 

ii 

Palladium    . 

Pd 

106 

Bromine 

Br     . 

79-9 

Phosphorus 

P 

31 

Cadmium 

Cd 

112 

Platinum 

Pt 

194-8 

Caesium 

Cs 

133 

Potassium       (ka- 

Calcium         .         . 

Ca 

40 

lium) 

K 

39'i 

Carbon  . 

C 

12 

Praseodymium     . 

Pr 

140 

Cerium. 

Ce 

140 

Rhodium 

Rh 

103 

Chlorine 

Cl 

35'5 

Rubidium    . 

Rb 

85-4 

Chromium     . 

Cr 

52-1 

Ruthenium  . 

Ru 

101-7 

Cobalt  . 

Co 

59 

Sa'marium    . 

Sa 

!50 

Copper  (cuprum) 

Cu 

63-6 

Scandium    . 

Sc 

44-1 

Erbium 

Er 

1  66 

Selenion 

Se 

Fluorine        .     ,    . 

F 

19 

Silicon 

Si 

28-4 

Gallium 

Ga 

70 

Silver  (argentum) 

Ag 

107-9 

Germanium  . 

Ge 

72 

Sodium  (natrium) 

Na 

23 

Gold  (aurum) 

Au 

197-2 

Strontium    . 

Sr 

87-6 

Helium 

He 

4 

Sulphur 

S 

32 

Hydrogen     .    *    . 

H 

i 

Tantalum 

Ta 

183 

Indium 

In 

114 

Tellurium     . 

Te 

127 

Iodine  .         . 

I 

126-8 

Thallium 

Tl 

204-1 

Iridium 

Ir 

193 

Thorium 

Th 

232 

Iron  (ferrum) 

Fe 

56 

Tin  (stannum) 

Sn 

118-5 

Krypton 

Kr 

Titanium 

T 

48-1 

Lanthanum  . 

La 

138 

Tungsten     (wolf- 

Lead (plumbum)  . 

Pb 

206-9 

ram) 

W 

184 

Lithium 

Li 

7 

Uranium 

U 

239'5 

Magnesium  . 

Mg 

24-4 

Vanadium    . 

V 

51-2 

Manganese    . 

Mn 

55 

Xeon    . 

Xe 

Mercury    (hydrar- 

Ytterbium   . 

Yb 

173 

gyrum)      . 

Hg 

200-3 

Yttrium 

Y 

89 

Metargon 

Mt 

— 

Zinc     . 

Zn 

65'4 

Molybdenum 

Mo 

96 

Zirconium    . 

Zr 

90-6 

FORMULA  OF   COMPOUNDS.  89 

The  formula  of  a  chemical  compound,  then,  tells  us 
not  only  of  what  elements  the  compound  consists — as 
these  elements  are  all  enumerated  in  the  formula — but 
also  how  much  of  each  element  is  contained  in  the 
compound.  We  are  now  able  to  see  of  what  great 
importance  these  formulae  are  in  all  chemical  inquiries. 

The  simple  calculations  which  we  shall  have  to 
make  will  quickly  convince  us  that  scientific  chemistry, 
and  therewith  the  only  way  of  studying  chemistry 
which  can  be  successful,  could  not  be  advanced  nowa- 
days without  employing  these  formulae,  which  are  based 
on  a  knowledge  of  the  atomic  weights  of  the  elements. 
It  is  to  be  expressly  remarked  that  all  such  calculations, 
in  so  far  as  we  shall  consider  them,  will  take  a  very 
simple  form.  Indeed,  it  will  not  be  necessary  to 
employ  anywhere  in  this  book  any  calculation  which 
demands  greater  arithmetical  knowledge  than  is 
possessed  by  everyone  who  has  attended  a  board 
school. 

The  first  formula  we  wrote  down  was  that  of  iron 
sulphide,  FeS.  At  that  time  the  formula  merely  in- 
formed us  that  the  substance  consists  of  iron  and 
sulphur.  We  now  know  that  the  formula  tells  much 
more  than  that ;  we  know  that  the  compound  is  not 
composed  of  any  indefinite  quantities  of  iron  and 
sulphur,  but  that  one  atom  of  iron  is  combined  therein 
with  one  atom  of  sulphur.  Moreover,  the  formula  tells 
that  56  parts  by  weight  of  iron  are  combined  with 
32  parts  by  weight  of  sulphur  in  iron  sulphide — the 
table  informs  us  that  the  atomic  weights  of  iron  and 
sulphur  are  56  and  32  respectively.  As  we  are  ac- 


9O       INTRODUCTION   TO   MODERN    CHEMISTRY. 

customed  to  reckon  in  grams,  the  ratio  of  the  weights 
will  be  expressed  in  the  clearest  way  by  using  the 
gram  as  unit  of  weight,  and  saying  that  32  grams  of 
sulphur  are  united  with  56  grams  of  iron  in  iron 
sulphide.*  If  we  melt  together  56  grams  of  iron  and 
32  grams  of  sulphur,  the  compound  iron  sulphide, 
FeS,  is  produced  directly.  But  if,  in  place  of  using 
56  grams  of  iron,  we  used  only  50  grams,  for 
32  grams  of  sulphur,  a  corresponding  quantity  of 
sulphur  would  remain  uncombined  with  iron  ;  for  some 
sulphur  would  lack  iron  wherewith  to  combine,  and 
that  sulphur  would  remain  over.  A  simple  proportion 
enables  us  to  calculate  what  that  quantity  of  sulphur 
must  be;  for,  as  56  parts  of  iron  require  32  parts  of 
sulphur,  we  have  merely  to  find  the  weight  of  sulphur, 
x,  required  by  50  parts  of  iron.  Here  is  the  calcu- 
lation : — 

56:32  =  50:*; 

therefore  x  =  5°  x  32  =  28-6. 

That  is  to  say,  50  grams  of  iron  combine  with  28-6 
grams  of  sulphur;  hence  it  follows  that  3-4  grams  of 
sulphur  (32  —  28*6)  remain  uncombined  when  50  grams 
of  iron  are  melted  with  32  grams  of  sulphur. 

Suppose  we  desire  to  know  the  quantities  of 
chlorine  and  sodium  which  form  common  salt.  The 
formula  of  common  salt  is  NaCl ;  the  atomic  weights  of 
chlorine  and  sodium  are  35-5  and  23  respectively,  and 

*  Of  course,  we  may  say  that  32  grains,  or  32  lb.,  or 
32  oz.  of  sulphur  are  combined  with  56  grains,  lb.,  or  oz. 
of  iron,  [TR.] 


FORMULA  OF   COMPOUNDS.  91 

the  sum  of  these  numbers  is  58*5  ;  therefore  58*5 
parts  by  weight — let  us  say  58-5  grams — of  common  salt 
contain  35*5  grams  of  chlorine.  Now  we  are  accus- 
tomed, for  the  sake  of  clearness,  to  calculate  all  such 
proportions  to  parts  per  hundred  ;  we  shall  therefore 
state  the  quantity  of  chlorine  in  common  salt  as  a 
percentage.  The  proportion  sum  is  simple  enough  : — 

58'5  :  35'5  =  I0°  :  x  (for  if  58'5  Parts  of  salt  contain  35-5  of 
chlorine,  x  parts  of  chlorine  will  be  contained  in  100  of  salt)  ; 

*5'i%  X  100 
hence  x  —  •"  *  =  607. 

5°'5 

| 

We  find,  then,  from  the  formula  of  common  salt, 
NaCl,  that  this  compound  contains  607  per  cent, 
of  chlorine.  As  a  man  consumes  about  25  grams 
[about  five-sixths  of  an  ounce]  of  common  salt  daily, 
he  takes  something  like  1 5  grams  [say  half  an  ounce] 
of  chlorine. 

By  the  method  illustrated  in  the  case  of  common  salt 
we  can,  of  course,  calculate  the  percentage  weight  of 
each  element  contained  in  any  compound  when  the 
formula  of  that  compound  is  known. 

As  water  is  a  substance  of  especial  interest  to  every- 
one, let  us  deduce  the  percentage  composition  of  that 
compound  from  its  formula.  The  formula  of  water  is 
H2O.  As  the  atomic  weight  of  hydrogen  is  I,  and  that 
of  oxygen  is  16,  the  sum  of  the  atomic  weights  in  this 
formula  is  18;  for  H2,  which  represents  2  atoms  of 
hydrogen,  weighs  2.  In  18  parts  by  weight — let  us 
say  in  18  grams — of  water  are  contained  2  parts 
by  weight — let  us  say  2  grams — of  hydrogen  and 


92        INTRODUCTION   TO   MODERN   CHEMISTRY. 

1 6   parts   by  weight — 16  grams — of  oxygen.      Calcu- 
lating to  percentages,  we  have  : — 

(i)  18  :    2  =  100  :  x  ;  hence  x  = 
(ii)  18  :  16  =  100  :  x ';  hence  x1  = 

That  is  to  say,  water  consists  of  irn   per  cent,  of 
hydrogen  and  88'89  Per  cent-  of  oxygen. 

It  is  not  to  be  expected  that  a  single  atom  of  one 
element  should  always  unite  with  a  single  atom  of 
another  element  to  form  a  compound.  The  formula 
which  has  just  been  given  for  water,  H2O,  shows  that 
two  atoms  of  hydrogen  unite  with  one  atom  of  oxygen 
to  produce  this  compound.  The  number  of  atoms 
of  the  individual  elements  that  enter  into  compounds 
varies  greatly.*  Although  it  is  evident,  from  what 
has  been  said  about  the  atomic  compositions  of  the 
elements,  that  these  bodies  must  always  combine  in 
atomic  proportions,  nevertheless  we  must  not  omit  to 
emphasise  this  fact  in  an  especial  way.  In  some  com- 
pounds we  find  I  atom  of  hydrogen,  in  others  20 
atoms  of  hydrogen  ;  we  find,  it  may  be,  5  atoms  of 

*  In  the  earlier  part  of  this  book,  for  the  sake  of  simplicity  and 
clearness,  we  used  the  abbreviated  forms  of  the  names  of  the 
elements  only  as  abbreviations  of  the  names — H,  for  instance, 
meant  hydrogen,  and  O  meant  oxygen.  Under  these  con- 
ditions it  was  not  possible  to  write  any  formula  wherein  more 
than  one  atom  of  an  element  occurred.  That  FeS  should 
signify  iron  sulphide  was  quite  apparent ;  but  why  should 
one  write  H2O,  for  instance,  if  H  signifies  hydrogen  and 
O  means  oxygen  ?  How  should  anyone  understand  this  H2 
as  long  as  he  is  unaware  that  the  abbreviation  H  represents 
one  atom  of  hydrogen  and  O  one  atom  of  oxygen  ?  If  the 


FORMULA  OF   COMPOUNDS.  93 

chlorine,  or  2  atoms  of  iron,  and  so  on  :  but  we  never 
find  ij  atoms  of  hydrogen,  or  I-J  atoms  of  chlorine,  or 
anything  of  that  kind;  for  such  a  state  of  affairs  is 
altogether  at  variance  with  the  idea  of  the  atom,  which 
signifies  indivisible.  Once  we  have  grasped  the  fact 
that  the  number  of  the  atoms  of  the  individual  elements 
that  enter  into  compounds  may  vary  in  a  remarkable 
way,  we  are  in  a  position  to  understand  those  formulae 
which  are  much  more  complicated  than  any  we  have 
yet  written  down.  To  take,  for  instance,  the  formula  of 
morphia,  which  is  C17H19NO3.  The  sum  of  the  atomic 
weights  is  easily  calculated.  As  the  weight  of  an  atom 
of  carbon,  C,  is  12,  C1T  amounts  to  17  x  12  =  204  ;  the 
19  atoms  of  hydrogen  weigh  19  (because  the  atomic 
weight  of  hydrogen  is  i)  ;  as  the  atomic  weights  of  nitro- 
gen and  oxygen  are  14  and  16  respectively,  evidently 
N  =  14,  and  O3  =  48.  The  sum  of  these  four  num- 
bers, 204  +  19+  14  +  48,  is  285.  Suppose  we  wish 
to  know  the  percentage  of  nitrogen  in  morphia ;  we 
find  it  by  the  proportion  sum : — 

14  x  100 
285  :  14  =  100  :  x\  x  =        2g$        =  4'92- 

The  percentages  of  carbon,  hydrogen,  and  oxygen  in 
morphia  are  found  by  a  similar  method  of  calculation. 

abbreviations  had  no  other  meaning  than  the  names  of  the 
elements,  the  expression  HO  would  suffice  to  represent  water, 
as  that  compound  consists  of  hydrogen  and  oxygen.  It  was 
necessary  to  use  formulae  very  sparingly  in  the  former  parts 
of  this  book,  as  we  were  in  a  position  to  employ  only  those 
compounds  which  consist  of  single  atoms  of  their  constituent 
elements,  considering  that  the  reader  did  not  know  that  the 
abbreviations  symbolised  atoms  of  the  various  elements,  and, 
therefore,  could  not  understand  the  meaning  of  the  small  figures 
attached  to  some  of  these  abbreviations. 


94       INTRODUCTION    TO   MODERN   CHEMISTRY. 

CALCULATING  FORMULAE  FROM  THE  RESULTS  OF 
ANALYSES. 

So  far  we  have  used  our  knowledge  of  atomic  weights 
to  calculate,  from  the  formula  of  a  compound,  the 
quantity  of  each  element  contained  in  100  parts  of 
that  compound.  If  we  think  over  the  matter  a  little, 
we  shall  see  that  we  may  begin  from  the  other  end,  so 
to  speak ;  for  hitherto  we  have  assumed  such  formulae 
as  those  of  water  and  morphia,  as  things  given  to  us. 
Formulae  are  not  at  all  such  things,  however.  It  is 
very  necessary  to  inquire  how  chemists  arrive  at  these 
formulae — how  they  are  able  to  deduce  the  formulae 
of  compounds  from  the  proportionate  weights  of  the 
elements  therein.  For  instance,  how  was  the  con- 
clusion arrived  at  that  morphia,  after  its  preparation 
from  opium  and  thorough  purification  by  re-crystal- 
lisation (this  was  done  in  1817),  is  composed  of  17 
atoms  of  carbon,  19  atoms  of  hydrogen,  I  atom  of 
nitrogen,  and  3  atoms  of  oxygen  ?  The  method  is  as 
follows. 

Every  chemical  compound  whose  formula  is  to  be 
established  is,  first  of  all,  analysed  qualitatively.  From 
this  we  learn  of  what  elements  the  substance  is  com- 
posed. Then  one  proceeds  to  a  quantitative  analysis — 
that  is  to  say,  a  determination  is  made,  by  the  use  of 
the  balance,  of  the  quantity  of  each  element  in  the 
compound.  As  the  methods  of  quantitative  analysis 
have  been  very  fully  developed,  this  is  not  generally  a 
very  difficult  task.  The  results  of  the  quantitative 


FORMULA  OF  COMPOUNDS.  95 

analysis  are  stated  in  percentages  of  each  element, 
and  the  formula  of  the  compound  is  arrived  at  by 
dividing  the  quantity  of  each  element  in  100  parts 
of  the  compound  by  the  atomic  weight  of  that 
element.  The  quotients  thus  obtained  are  propor- 
tional to  the  numbers  of  the  atoms,  and  from  these 
quotients  the  numbers  of  atoms  are  easily  found. 
This  statement  sounds  more  complicated  than  the 
affair  really  is  ;  a  few  examples  will  best  elucidate 
the  matter. 

A  quantitative  analysis  of  water,  made  by  a  method 
which  we  shall  very  soon  become  acquainted  with, 
shows  that  water  is  composed  of  1 1  •  1 1  per  cent,  hydro- 
gen and  88*89  per  cent,  oxygen.  To  get  this  length 
requires  the  balance,  but  not  chemical  formulae.  How 
does  one  pass  from  this  result  to  the  formula  of 
water,  H2O  ?  For  this  purpose  the  atomic  weights  of 
the  elements  are  divided  into  the  percentage  quanti- 
ties of  these  elements.  As  the  atomic  weight  of 
hydrogen  is  I,  the  number  irn  remains  unchanged. 
The  quotient  obtained  by  dividing  88*89  by  16  (which 
is  the  atomic  weight  of  oxygen)  is  5 '5 5-  The  ratio  of 
the  numbers  of  atoms  of  hydrogen  to  that  of  the  atoms 
of  oxygen  is,  therefore,  as  irn  :  5'55>  or  as  2  '•  *• 
In  water,  then,  there  are  always  present  two  atoms  of 
hydrogen  to  one  atom  of  oxygen ;  therefore  we  write 
the  formula  of  water  as  H2O. 

As  a  second  example  we  shall  take  the  calculation  of 
the  formula  of  iron  oxide,  a  substance  so  often  seen  on 
vessels  made  of  iron.  The  quantitative  analysis  of  this 
substance,  made  by  using  the  balance,  tells  that  it 
consists  of  70  per  cent,  of  iron  and  30  per  cent,  of 


96       INTRODUCTION   TO   MODERN   CHEMISTRY. 

oxygen.  Dividing  these  percentages  by  the  atomic 
weights,  56  for  iron  and  16  for  oxygen,  we  have  these 
results  :  — 


Looking  at  the  ratio  1*25  :  1*875,  we  see  ^a^  ^  *s  as 
2  :  3.  For  two  atoms  of  iron,  then,  there  are  always 
three  atoms  of  oxygen  in  this  compound  ;  hence  the 
formula  of  this  oxide  of  iron  is  Fe2O3. 

Our  third  example  shall  be  a  more  complicated  one. 
We  will  bring  morphia  again  under  consideration. 
Here  are  the  results  of  the  quantitative  analysis  of  this 
compound  :— 

Carbon    .  71-58  per  cent.  Atomic  weight  of  carbon  (C)      =  12. 

Oxygen    .  16-84        .,  „              „  ,       oxygen  (O)     =16. 

Hydrogen    6*66        „  „              „          hydrogen  (H)  =   i. 

Nitrogen.     4-92        ,,  „              „          nitrogen  (N)    =14. 

Let  us  divide  these  percentage  quantities  by  the 
atomic  weights  of  the  elements.  The  results  are 
these  :— 

71  5     _  5.97  for  carbon  ;        —  -^  =  1*05  for  oxygen  ; 
5^  =  6-66  for  hydrogen  ;    ^       =  -35  for  nitrogen. 


The  numbers  of  the  atoms  of  the  four  elements  whereof 
morphia  is  composed  are  in  the  ratio  5*97  :  1-05  :  6*66  : 

•35- 

If  we  calculate  these   numbers  to  whole  numbers, 

assuming  that  there  is  in  the  compound  only  one  atom 


.      ATOMIC   WEIGHTS.  97 

of  that  element  which  is  present  in  the  smallest  quantity, 
we  arrive  at  this  result  :  — 

(i)  -35  :  I  =  5-97  :  x\    x  ^        ^'  =  almost  exactly*  17. 
(ii)  -35  :  I  =  1-05  :  y  ;   y  =  l~~~^  =       „  „          3- 

:i-  6-66  :  z  ;    *  =          ^  -       „        :  .  „        19. 


Hence  morphia  is  composed  of  17  atoms  carbon, 
3  atoms  oxygen,  19  atoms  hydrogen,  and  I  atom 
nitrogen  ;  in  other  words,  the  formula  of  morphia  is 
C17H19N03. 

The  calculation,  and  hence  the  determination,  of  the 
formula  of  a  compound  is  scarcely  rendered  more 
difficult  should  there  be  more  than  a  single  atom  of 
each  of  the  elements  that  form  the  compound,  and 
not,  as  in  the  case  of  morphia,  only  a  single  atom  of 
one  of  these  elements. 

As  an  example  of  a  case  of  this  kind,  let  us  take 
benzoic  acid,  the  compound  we  used  to  illustrate  the 
process  of  crystallisation.  The  quantitative  analysis  of 
this  compound  shows  that  it  is  composed  of  68*85  Per 
cent,  carbon,  26*23  per  cent,  oxygen,  and  4*92  per  cent. 
hydrogen.  Dividing  these  numbers  by  the  atomic 

68-85 
weights,  we  obtain  for  carbon  -  -  =  574,  for  oxygen 

'—£--  —  1*64,    and    for   hydrogen   zlL  =  4-92.      The 

*  The  reason  for  the  slight  divergences  of  the  numbers  from 
whole  numbers  is  that  we  have  used  whole  numbers  as  values  for 
the  atomic  weights.  The  atomic  weight  of  carbon  is  1  1  -97,  not 
12,  and  so  on. 


98        INTRODUCTION    TO   MODERN    CHEMISTRY. 

ratio  of  the  numbers  of  atoms  in  this  case  is,  then, 
574  :  1*64  :  4-92.  An  examination  of  these  figures 
shows  that  they  are  in  the  same  ratio  as  3^  :  I  :  3. 
But  we  cannot  have  half-atoms ;  therefore,  to  get  whole 
numbers,  we  multiply  all  these  figures  by  2,  and  obtain 
the  ratio  7  :  2  :  6  as  that  which  the  numbers  of  atoms 
bear  to  one  another  :  in  other  words,  we  arrive  at  the 
formula  C7O2H6  for  benzoic  acid. 

We  have  now  examined  sufficiently  the  method 
whereby  the  formulae  of  compounds  are  deduced  from 
the  results  of  the  quantitative  analyses  of  these  com- 
pounds (made  by  employing  the  balance)  with  the  aid 
of  the  atomic  weights  of  the  elements.  For  even  if  the 
numbers  of  the  atoms  are  in  a  more  complex  ratio  than 
is  shown  in  any  of  our  examples,  it  is  evidently  always 
possible  to  state  the  ratio  in  whole  numbers  without 
much  difficulty. 


MOLECULES   AND   THEIR   WEIGHTS. 

THE  conception  that  all  bodies  are  composed  of  the 
smallest  particles — that  is,  of  atoms — has  sufficed  for 
philosophy  for  two  thousand  years.  It  has  not  sufficed 
for  chemistry.  The  untenability  of  the  hypothesis  that 
the  smallest  particle  of  every  substance  around  us  is 
an  atom  is  demonstrated  very  simply  by  the  following 
considerations.  We  may  certainly  very  well  suppose 
that  the  smallest  particle  of  an  element— say  of  iron, 
lead,  chlorine,  or  hydrogen — is  an  atom,  in  the  sense 
wherein  we  have  hitherto  always  used  this  word ;  but 
what  about  the  smallest  particle  of  a  compound — for 
instance,  of  that  compound  which  we  have  so  often 
spoken  about,  hydrochloric  acid  ?  The  smallest  particle 
of  that  body  always  consists  of  two  atoms  of  the 
elements  whereof  the  body  is  composed  :  it  consists  of 
one  atom  of  hydrogen  and  one  atom  of  chlorine,  it  is 
a  complex  of  two  atoms,  and  it  is  therefore  certain  that 
this  smallest  particle  is  not  indivisible — that  it  is  not  an 
atom.  And  as  with  this,  so,  of  course,  with  other 
compounds.  The  smallest  particle  of  a  compound 
must  always,  and  self-evidently,  consist  of  atoms  of 
the  elements  which  compose  that  compound  :  it  must 
always  be  a  cluster  of  atoms.  As  the  designation  atom 
is  impossible,  indeed  meaningless,  if  applied  to  the 

99 


100      INTRODUCTION   TO   MODERN   CHEMISTRY. 

smallest  particles  of  compound  bodies,  it  is  necessary 
to  have  a  special  name  for  these  particles.  They  are 
called  molecules.  The  smallest  particle  of  hydrochloric 
acid  is,  then,  a  molecule  of  hydrochloric  acid  ;  the 
smallest  particle  of  common  salt — sodium  chloride — 
is  a  molecule  of  sodium  chloride.  These  two  mole- 
cules both  consist  of  two  atoms.  But  molecules 
may  be  very  large  as  compared  with  atoms.  Think 
of  morphia,  for  example.  The  formula  C17H19NO3 
shows  that  the  molecule,  the  smallest  particle  we 
can  think  about,  of  this  compound  consists  of  forty 
atoms. 

We  now  know  something  of  the  signification  of  the 
expression  molecule.  We  will  proceed  to  a  further 
consideration  of  molecules  and  their  weights.  If  the 
reader  has  got  a  firm  hold  of  the  conception  of  the 
molecule  from  the  statements  which  have  been  given,  he 
will  certainly  be  able  to  follow  the  later  parts  of  this 
book  without  those  more  difficult  deductions  about  the 
weights  of  molecules  which  we  are  about  to  consider. 
On  that  account  let  us  recapitulate  in  a  word.  An 
atom  is  the  smallest  part  of  an  element ;  a  molecule  is 
the  smallest  part  of  a  compound  body. 

The  word  molecule  is  derived  from  the  Latin  :  mole- 
cula  means  a  small  mass.  The  detailed  statements 
concerning  molecules  will  be  a  little  more  complicated 
than  those  concerning  atoms  and  atomic  weights. 
These  statements  form  the  chapter  of  this  book  which 
requires  more  attention  than  any  other.  All  the  later 
considerations  are  easily  to  be  understood  if  the  hypo- 
thesis of  the  existence  of  atoms  and  molecules  is 
granted. 


MOLECULAR   WEIGHTS.  IOI 

The  few  fundamental  considerations  which  we  have 
found  it  necessary  to  lay  down  in  our  development  of 
the  hypothesis  of  atoms  and  the  weights  of  atoms 
were  grounded,  essentially,  on  the  behaviour  of  hydro- 
gen towards  chlorine,  bromine,  and  iodine.  For  this 
reason  we  have  dwelt  somewhat  fully  on  the  ex- 
perimental examination  of  the  compounds  of  these 
element?.  For  our  treatment  of  the  development  of 
the  atomic  theory,  the  fact  is  of  the  utmost  importance 
that  only  a  single  compound  of  hydrogen  with  each 
of  these  elements  has  been  obtained — namely,  hydro- 
chloric acid  (HC1),  hydrobromic  acid  (HBr),  and 
hydriodic  acid  (HI) — notwithstanding  the  many  at- 
tempts that  have  been  made  to  form  more  compounds 
than  these. 

In  deducing  atomic  weights,  we  have  made  calcula- 
tions concerning  the  quantities  by  weight  of  hydrogen, 
chlorine,  bromine,  and  iodine  contained  in  these  gases 
as  if  we  were  dealing  with  solid  bodies  ;  we  have  paid 
no  heed  in  our  calculations  to  the  fact  that  we  have 
had  to  do  with  gases.  Now,  however,  we  shall  make 
up  for  that  neglect.  Let  us  begin  by  using  the  balance 
and  proceeding  as  we  did  in  deducing  atomic  weights ; 
let  us  keep  away  from  every  theory  until  we  have  estab- 
lished facts.  The  result  of  determining  the  specific 
gravity  of  chlorine  is  that  this  gas  is  35-5  times  heavier 
than  an  equal  volume  of  hydrogen.  By  heating 
bromine  we  change  that  liquid  element  into  a  gas ;  then, 
determining  the  specific  gravity  of  the  gas,  we  find  that 
it  is  80  times  heavier  than  hydrogen  at  the  same 
temperature ;  in  other  words,  the  specific  gravity  of 
bromine  gas  is  80.  Proceeding  in  a  similar  way,  we 


102      INTRODUCTION    TO   MODERN    CHEMISTRY. 


find  the  specific  gravity  of  iodine  gas  to  be  127.  Let 
us  now  place  the  values  we  found  for  the  atomic  weights 
of  the  four  elements  side  by  side  with  those  we  have 
just  established  for  the  specific  gravities  of  the  same 
elements  in  the  state  of  gas.  A  very  striking  fact  is 
apparent. 


Name  of  Element. 

Atomic  weight 
of  Element. 

Specific  gravity 
of  Element  in 
state  of  gas. 

Hydrogen.         .        ... 

I 

I 

Chlorine    .         . 

35'5 

35-5 

Bromine    . 

80 

80 

Iodine        . 

127 

127 

The  table  shows  that  the  numbers  which  express  the 
atomic  weights  of  these  elements  are  the  same  as  those 
which  express  the  specific  gravities  of  the  elements  in  the 
gaseous  state. 

The  remarkable  and  astonishing  agreement  between 
these  two  sets  of  numbers  could  not  have  been  fore- 
seen, and  could  not  have  been  established  except  by 
using  the  balance.  This  agreement,  which  was  known 
first  in  the  early  years  of  the  nineteenth  century,  is  in 
keeping  with  the  general  behaviour  of  all  gases  which 
have  been  examined  by  physicists  during  the  last  two 
hundred  years — that  is,  long  before  chemists  had  de- 
veloped their  theory  of  atoms  and  molecules.  Physicists 
established  the  fact  that  all  gases,  and  all  chemical  com- 
pounds in  the  state  of  gas — water-gas,  for  instance — 
have  certain  properties  which  are  common  to  all  of 
them.  For  instance,  they  behave  similarly  towards 


MOLECULAR   WEIGHTS.  103 

temperature  and  pressure  ;  that  is  to  say,  all  gases 
or  gasified  compounds  are  equally  compressed  by 
equal  pressures,  expand  equally  by  equal  increments 
of  temperature,  and  contract  to  the  same  extent  by 
equal  lowerings  of  temperature. 

Inasmuch  as  this  statement  holds  good  for  all  gases, 
and  for  all  chemical  compounds  when  gasified — for 
chlorine  gas  and  bromine  gas  as  for  water-gas  and 
hydrochloric  acid  gas — it  is  evident  that  the  chemical 
compositions  of  gaseous  bodies  have  no  influence  on 
their  behaviour  towards  pressure  and  temperature, 
which  is  always  the  same  for  all  gases.  A  considera- 
tion of  this  uniform  behaviour  of  all  gases  shows  that, 
although  gases  may  differ  radically  in  chemical  com- 
position— what  chemical  likeness  is  there,  for  instance, 
between  chlorine  gas  and  water-gas  ? — yet  they  must 
all  have  something  in  common.  This  common  some- 
thing cannot  have  to  do  with  the  chemical  relations 
of  the  gases. 

Careful  reflection  at  last  led  to  the  conclusion  that 
the  only  way  of  making  intelligible  this  behaviour, 
which  is  common  to  all  gases,  was  by  supposing  that 
equal  volumes — say  one  litre — of  all  gases  contain 
equal  numbers  of  smallest  particles,  whether  these 
smallest  particles  be  particles  of,  let  us  say,  chlorine, 
or  water-gas,  or  hydriodic  acid  gas.  This  conclusion 
seems  forced,  but  no  serious  objection  has  been  found 
against  it ;  it  has  been  arrived  at,  not  only  by  chemists, 
but  also  by  physicists,  from  a  study  of  the  behaviour 
of  gases,  for  all  the  phenomena  of  gases  which  are 
studied  in  physics  compel  us  to  suppose  that  equal 


104      INTRODUCTION   TO   MODERN   CHEMISTRY. 

numbers  of  smallest  particles  are  contained  in   equal 
volumes  of  all  gases. 

Suppose  we  assume,  for  instance,  that  there  are 
a  hundred  million  smallest  particles  in  one  litre  of 
hydrogen  gas,  a  hundred  million  smallest  particles  in 
one  litre  of  chlorine  gas,  and  a  hundred  million  smallest 
particles  in  one  litre  of  bromine  gas ;  then  we  can 
readily  understand  that  these  three  gases,  although 
they  are  certainly,  in  a  chemical  sense,  exceedingly 
unlike  one  another,  for  each  is  a  particular  element, 
nevertheless  are  equally  compressed  by  equal  pressures 
and  expand  equally  for  equal  increments  of  tempera- 
ture. But  if  we  make  the  supposition  that  a  litre  of 
hydrogen  contains,  say,  a  hundred  and  ten  million 
smallest  particles,  a  litre  of  chlorine  gas  ninety  million, 
and  a  litre  of  bromine  gas  eighty-five  million,  smallest 
particles,  then  there  is  an  end  to  all  possibility  of 
conceiving  how,  under  these  circumstances,  the  three 
gases  should  behave  in  the  same  way  towards  pressure 
and  temperature. 

It  is  certainly  not  difficult  to  follow  this  argument, 
which  is  of  a  negative  character  it  is  true,  for  the 
existence  of  an  equal  number  of  smallest  particles  in 
equal  volumes  of  all  gases.  But  the  facts  we  have 
now  to  consider  seem  to  be  in  complete  contradiction 
to  our  supposition  that  equal  numbers  of  smallest 
particles  are  contained  in  equal  volumes  of  all  gases. 
The  elucidation  of  this  apparent  impossibility — that  is, 
the  clear  setting  forth  of  the  reasons  for  it — has  caused 
endless  trouble. 


MOLECULAR   WEIGHTS.  IO5 

Let  us  make  a  practical  application  of  our  supposition. 
Let  us  cause  one  volume  of  hydrogen  gas,  containing, 
let  us  say,  one  thousand  smallest  particles,  to  unite 
with  an  equal  volume  of  chlorine  gas,  which  will  also 
contain  one  thousand  smallest  particles.*  We  should 
expect  to  obtain  one  volume  of  hydrochloric  acid  gas, 
which  must  also  contain  one  thousand  smallest  particles 
of  hydrochloric  acid ;  for  equal  numbers  of  smallest 
particles  are  contained,  according  to  our  supposition, 
in  equal  volumes  of  all  gases.  To  make  the  quantities 
such  as  may  be  most  conveniently  thought  of  by  us, 
let  us  suppose  we  take  one  litre  of  each  gas ;  then 
one  litre  of  hydrogen  and  one  litre  of  chlorine  should 
yield  one  litre  of  hydrochloric  acid  gas.  Now,  if  the 
experiment  is  actually  conducted,  we  obtain,  not  one  litre, 
but  two  litres,  of  hydrochloric  acid  gas. 

To  sum  up.  This  incontrovertible  experimental  result, 
which  is  always  obtained  when  the  experiment  is 
repeated,  seems  to  be  in  complete  contradiction  to  our 
assumption  of  the  equality  of  the  numbers  of  smallest 
particles  in  equal  volumes  of  all  gases.  The  contra- 
diction is,  however,  only  apparent.  It  is  we  who  are 
responsible  for  the  contradiction  ;  for  we  have  confused 
things  which  have  nothing  to  do  with  one  another, 
although  it  is  true  that  the  matter  is  a  difficult  one ; 
and  it  is  this  difficulty  which  accounts  for  the  long 
time  that  was  required  to  make  it  clear.' 

The  smallest  particle  of  hydrogen,  H,  is  an  atom ; 
the  smallest  particle  of  chlorine,  Cl,  is  also  an  atom  ; 

*  As  we  have  already  learned  (p.  47)  that  these  gases  combine 
in  sunshine,  we  know  how  the  experiment  may  be  conducted, 


106      INTRODUCTION    TO   MODERN    CHEMISTRY. 

but  is  the  smallest  particle  of  hydrochloric  acid,  HC1, 
also  an  atom  ?  No,  that  particle  is  not  an  atom,  for 
it  consists  of  two  atoms  ;  as  we  know  already,  that 
particle  is  a  molecule.  Now  the  contradiction  begins 
to  be  cleared  up.  We  have  been  trying  to  com- 
pare with  one  another  smallest  particles  which  are 
not  comparable  ;  for  there  are  two  kinds  of  smallest 
particles,  and  these  are  quite  different  one  from 
another. 

But  how  will  keeping  definitely  apart  the  conceptions 
of  the  atom  and  the  molecule  enable  us  to  explain  the 
apparent  contradiction  in  the  behaviour  of  the  gases 
we  have  been  examining  ?  The  following  considerations 
will  be  helpful. 

We  must  certainly  ascribe  to  the  atoms  of  every 
element  a  tendency  to  combine  with  atoms  of  many 
other  elements.  If  this  were  not  so,  the  elements 
would  remain  for  ever  as  such,  side  by  side,  and  no 
compounds  would  be  formed  between  them.  What 
becomes  of  this  tendency  of  the  atoms  when  the 
elements  make  their  appearance  as  such  ?  For  in- 
stance, what  becomes  of  this  tendency  when  hydrogen 
gas  or  chlorine  gas  is  prepared  ?  We  must  suppose 
that  in  the  cases  of  these  elementary  gases — or,  to 
express  it  otherwise,  in  these  gaseous  elements — this 
tendency  of  the  atoms  to  combine  finds  its  outcome  in 
binding  together  the  atoms  of  these  gaseous  elements 
(since  there  are  no  atoms  at  hand  of  other  elements 
wherewith  the  atoms  in  question  might  perhaps  more 
willingly  combine),  so  that  in  these  gases  two  atoms 
are  joined  together  to  form  a  molecule. 


MOLECULES   AND   ATOMS. 


10; 


The  following  is  an  argument  which  may  be  adduced 
in  favour  of  what  we  have  stated  to  be  the  behaviour 
of  the  atoms  in  the  elementary  gases.  At  the  moment 
when  they  are  set  free,  when  they  must  be  considered 
[by  our  supposition]  to  exist  as  atoms,  the  be- 
haviours of  the  elementary  gases  differ  from  their 
behaviours  when  we  see  them  as  completely  formed 
gases,  when  their  atoms  have  had  time  to  combine 
with  one  another  and 
form  molecules. 

The  following  de- 
monstration, which  is 
easily  followed,  will 
be  especially  service- 
able. The  reaction  of 
zinc  and  diluted  sul- 
phuric acid  produces 
hydrogen  gas  (p.  34). 
We  shall  make  use 
of  that  reaction.  We 
put  the  two  substances 
into  the  flask  A,  and 
also  into  the  flask  B  (fig.  28),  and  we  pass  the  gas 
from  A  into  a  liquid  called  nitrobenzene,  contained  in 
the  vessel  c.  Nitrobenzene  is  a  compound  of  6  atoms 
of  carbon,  5  atoms  of  hydrogen,  I  atom  of  nitrogen, 
and  2  atoms  of  oxygen  :  its  formula  is  C6H5NO2.  For 
however  long  a  time  we  pass  hydrogen  gas  through 
nitrobenzene,  in  the  manner  of  this  experiment,  the 
nitrobenzene  remains  unchanged.  But  quite  a  differ- 
ent result  is  obtained  if  we  pour  some  nitrobenzene 
(through  the  funnel  D  into  the  flask  B,  fig.  28)  directly 


Fig.  28. — Experiments  to  illustrate 
reactions  of  nascent   hydrogen. 


108      INTRODUCTION    TO   MODERN    CHEMISTRY. 

into  a  mixture  of  zinc  and  diluted  sulphuric  acid, 
although  the  substances  employed — zinc,  diluted  sul- 
phuric acid,  and  nitrobenzene — are  the  same  as  before. 
The  inclination  of  hydrogen  to  form  water  by  com- 
bining with  oxygen  now  comes  into  play.  The  whole 
of  the  oxygen  of  the  nitrobenzene  combines  with 
hydrogen,  produced  in  the  flask  B,  to  form  water,  and 
is  thus  removed  from  the  nitrobenzene.  Two  atoms  of 
hydrogen — the  source  of  which  is,  of  course,  the  zinc 
and  sulphuric  acid — take  the  place  of  the  oxygen. 
The  body  which  is  thus  formed  from  nitrobenzene, 
and  is  found  in  the  flask  B  in  the  place  of  the  nitro- 
benzene, is  aniline,  the  mother-substance  of  the  aniline 
colours.  The  following  equation  expresses  the  change 
of  nitrobenzene,  CCH5NO2,  into  aniline,  C6H5NH2 : — 

C6H5NO,     +       6H  C6H5NH2  +  2H2O. 

Nitrobenzene  +  hydrogen  =    aniline    +  water. 

The  foregoing  experiment  shows  that  hydrogen  at 
the  moment  of  its  formation — in  the  nascent  state,  as 
the  phrase  is — reacts  with  nitrobenzene  differently 
from  the  manner  wherein  it  reacts  when  it  is  made  and 
then  passed  into  nitrobenzene.  Many  other  bodies  show 
reactions  like  this  of  nitrobenzene,  under  corresponding 
conditions.  For  the  following  reason?,  these  facts  are 
explicable  only  in  terms  of  the  hypothesis  we  have 
made  that  molecules,  not  free  atoms,  are  present  in  ele- 
mentary gases  when  these  exist  as  such.  When  zinc 
and  diluted  sulphuric  acid  are  brought  into  contact  with 
one  another,  hydrogen  is  formed,  and  in  the  first 
moment  of  its  formation  that  gas  must  surely  be  pro- 
duced atom  by  atom.  If  nitrobenzene  is  added  to  this 


MOLECULES  AND  ATOMS.  109 

mixture,  the  single  atoms  of  hydrogen  rush  upon  the 
oxygen  of  the  nitrobenzene  with  all  the  tendency  to 
combine  which  is  naturally  inherent  in  them,  and, 
uniting  therewith,  form  water.  In  the  nascent  state 
(or,  one  may  say,  at  the  moment  of  their  birth)  these 
atoms  act  successfully.  But  if  the  hydrogen  gas  is 
allowed  to  escape  as  such  from  the  mixture,  the  two 
atoms  of  hydrogen  combine  together  to  form  a  molecule, 
each  thereby  satisfying  part  of  the  tendency  to  combine 
of  the  other,  and  so  using  up  some  of  their  energy  of 
combination.  Hence  the  energy  remaining  in  such  a 
molecule  of  hydrogen  is  not  sufficient  to  drag  away  the 
oxygen  from  nitrobenzene  when  the  hydrogen  comes 
into  contact  with  that  compound  ;  and  so  it  is  that 
hydrogen  gas,  in  contradistinction  to  hydrogen  in  the 
nascent  state,  does  not  change  nitrobenzene,  as  was 
shown  in  our  experiment.  As  gaseous  hydrogen  is 
not  able  to  perform  actions  which  hydrogen  in  the 
nascent  state  can  perform,  there  cannot  be  any  single 
atoms  present  in  hydrogen  gas  existing  as  such.  Part 
of  the  energy  of  the  atoms  must  have  been  satisfied  by 
their  mutual  combination  :  they  must  have  united  to 
form  molecules. 

Hence  there  are  no  single  atoms  in  the  elementary 
gases — or,  to  express  it  otherwise,  in  the  gaseous 
elements — such  as  hydrogen  gas  or  chlorine  gas ;  but 
two  atoms  are  always  combined  in  these  gases,  to 
form  a  molecule  of  hydrogen,  H2,  or  a  molecule  of 
chlorine,  Clz. 

Now,  if  in  the  elementary  gases,  such  as  hydrogen 
gas  and  chlorine  gas,  there  are  no  free  single  atoms, 


I  10      INTRODUCTION   TO   MODERN    CHEMISTRY. 

but  these  are  united  in  pairs,  so  forming  molecules,  we 
are  in  a  position  to  explain  the  apparent  contradiction 
(with  which  we  are  now  dealing) — namely,  that  one 
volume  of  hydrogen  gas  and  one  volume  of  chlorine  gas 
produce  two  volumes  of  hydrochloric  acid  gas.  The 
explanation  is  this  : — The  thousand  smallest  particles 
contained  in  one  volume  of  hydrogen  are  not,  as  we 
before  assumed,  a  thousand  atoms  of  hydrogen,  H,  but 
they  are  a  thousand  molecules,  H2 ;  and  an  equal 
volume  of  chlorine  gas  contains,  not  a  thousand  atoms 
of  chlorine,  Cl,  but  a  thousand  molecules,  C12.  When 
these  two  volumes  combine,  they  produce,  of  course, 
two  thousand  smallest  particles — that  is,  two  thousand 
molecules — of  hydrochloric  acid. 

icooH2  +  loooCl,  =  2oooHCl. 

The  theory  of  gases,  which  states  that  equal  volumes 
of  all  gases  contain  equal  numbers  of  smallest  particles, 
shows  that  two  thousand  smallest  particles — that  is, 
two  thousand  molecules — of  any  gas  occupy  twice  the 
volume  of  one  thousand  molecules  of  any  other  gas. 
Hence  the  two  thousand  molecules  of  hydrochloric  acid, 
HC1,  occupy  twice  the  space  occupied  by  a  thousand 
molecules  of  hydrogen,  H2,  or  a  thousand  molecules  of 
chlorine,  C12.  We  must,  therefore,  obtain  two  litres 
of  hydrochloric  acid  gas  from  one  litre  of  hydrogen 
gas  and  one  litre  of  chlorine  gas,  as  we  found  to  be 
the  case  in  that  experiment  which  at  first  seemed 
to  yield  an  impossible  result.  The  explanation  of 
that  result  is  seen  to  be  impossible  so  long  as  the 
conceptions  of  the  atom  and  the  molecule  are  not 
sharply  differentiated. 


DETERMINATION   OF   MOLECULAR   WEIGHTS      III 

The  combination  of  a  thousand  molecules  of  hydro- 
gen, H2,  with  a  thousand  molecules  of  chlorine,  C12, 
to  produce  two  thousand  molecules  of  hydrochloric 
acid  gas,  HC1,  may  perhaps  be  made  clearer  to  many 
by  the  following  illustration.  Imagine  a  thousand  male 
twins  and  a  thousand  female  twins  —  these  would 
represent  the  molecules  H2  and  C12;  let  them  all 
marry  with  one  another ;  there  will  be  two  thousand 
married  couples,  corresponding  with  the  two  thousand 
molecules,  HC1. 

What  we  have  demonstrated  for  hydrochloric  acid 
gas  holds  good,  of  course,  for  all  bodies  in  the  gaseous 
state. 

The  weight  of  the  molecule  of  a  substance  in  the  state  of 
gas  can  be  found  from  the  specific  gravity  of  that  gas, 
because  equal  numbers  of  molecules  are  contained  in  equal 
volumes  of  all  gases.  Let  us  take  water  as  an  example. 
The  specific  gravity  of  water-gas  (water  above  100°  C.) 
[212°  F.]  is  9,  referred  to  hydrogen  as  unity.  A  litre 
of  water-gas  is  nine  times  heavier  than  a  litre  of 
hydrogen  at  the  same  temperature.  Now  we  must 
compare  like  with  like ;  and  as  water-gas  consists  of 
molecules,  we  must  compare  it  with  the  molecule  of 
hydrogen,  H2.  Hence  the  molecular  weight  is  twice 
the  specific  gravity  of  the  gas — that  is,  2  X  9  =  1 8 ;  and 
the  formula  H2O  (which  represents  the  weight  18) 
corresponds  with  this  result.  The  weight  of  the  mole- 
cule of  any  gasifiable  chemical  compound  may  be 
determined  in  this  way.  Such  determinations  are  of 
great  importance  in  scientific  work — for  instance,  when 
one  has  to  deal  with  a  newly  discovered  substance. 


112      INTRODUCTION   TO   MODERN   CHEMISTRY. 

The  main  result  of  our  considerations  re- 
garding atoms  and  molecules  is,  then,  the 
following.  Atoms  are  the  smallest,  indivi- 
sible particles  of  elements,  molecules  the 
smallest,  divisible  particles  of  compounds. 
It  is  possible,  by  using  the  balance,  to  de- 
termine the  weights  both  of  atoms  and 
molecules,  the  weight  of  an  atom  of  hydrogen 
being  taken  as  unity. 

We  now  proceed  once  more  to1  the  acquisition  of 
purely  chemical  knowledge,  which  will  gradually  lead 
on  to  new  general  conclusions.  We  shall  begin  by 
turning  our  attention  to  oxygen. 


OXYGEN. 

ONE  of  our  first  experiments  was  to  prepare  oxygen 
gas  by  decomposing  mercury  oxide  into  its  components. 
In  connection  with  that  method  we  explained  what  is 
meant  by  analysis.  But  the  method  is  not  a  good  one 
for  preparing  oxygen,  because  the  quantity  of  oxygen, 
proportional  to  that  of  mercury,  in  mercury  oxide  is 
small.  Let  us  calculate  the  quantity  of  oxygen  in  the 
compound.  The  formula  of  mercury  oxide  is  HgO. 
The  atomic  weights  of  mercury  and  oxygen  respec- 
tively are  200  and  16.  Hence  216  parts  by  weight — 
let  us  say  216  grams — of  mercury  oxide  contain  j6 
grams  of  oxygen ;  therefore  (from  the  proportion 
216  :  16  =  100  :  x)  the  percentage  of  oxygen  is  7*4 

only- 
There  is  a  salt  known  in  commerce  as  chlorate  of 
potash  (it  is  more  accurately  called  potassium  chlorate), 
which  is  very  rich  in  oxygen,  and,  like  mercury  oxide, 
yields  the  whole  of  its  oxygen  when  it  is  heated.  We 
shall  learn  how  this  salt  is  made  at  a  later  time; 
meanwhile,  as  it  can  be  dealt  with  as  conveniently  as 
mercury  oxide,  we  shall  make  use  of  it.  The  formula 
of  potassium  chlorate  is  KC1O3.  As  the  atomic  weight 
of  potassium,  K,  is  39,  that  of  chlorine,  Cl,  is  35-5,  and 
that  of  oxygen,  O,  is  16,  and  as  there  are  three  atoms 

"3  8 


114      INTRODUCTION    TO   MODERN   CHEMISTRY. 

of  oxygen  in  the  compound,  the  molecular  weight  of 
the  salt  is  122-5. 

K        Cl          03. 

39 +  35'5 +  (3  x  16)=  122-5. 

In  1 22-5  grams  of  potassium  chlorate  there  are  48 
grams  of  oxygen  ;  this  is  equal  to  39-2  per  cent,  of 
oxygen,  which  is  fully  five  times  more  than  the 
percentage  of  oxygen  in  mercury  oxide. 

This  time  we  shall  collect  our  oxygen  in  a  gasholder. 
The  arrangement  of  such  a  gasholder  as  is  commonly 
used  in  the  laboratory  is  shown  in  fig.  29.  This 
apparatus,  which  is  evidently  very  different  from  the 
gasometers  used  in  gas-works,  consists  of  a  lower  vessel 
(A)  holding  about  15  litres  [say,  4  gallons],  connected 
with  an  upper  vessel  (B).  The  lower  vessel  is  closed 
air-tight,  and  a  tube  (c)  with  a  stopcock  (K)  passes  from 
the  upper  vessel  to  the  bottom  of  the  lower  one.  There 
is  also  an  opening  (D)  near  the  bottom  of  A  ;  this  open- 
ing can  be  closed  by  a  screw-cap  (E).  The  top  of  the 
lower  vessel  carries  a  second  tube,  provided  with  a 
stopcock  (F)  and  connected  with  a  tube  drawn  to  a  fine 
opening  (H)  :  L  serves  as  a  third  support  for  the  upper 
vessel  B.  If  the  reservoir  B  is  filled  with  water,  and 
the  stopcocks  K  and  F  are  opened,  water  will  flow  into 
A,  and  the  air  in  that  vessel  will  escape  through  H 
until,  by  using  sufficient  water,  the  lower  vessel  is 
filled  with  water.  While  this  is  being  done,  the  open- 
ing D  is,  of  course,  closed  by  the  screw-cap  E.  If  both 
stopcocks  are  now  closed,  and  DIS  set  open  (by  unscrew- 
ing the  cap  E),  the  water  in  A  will  not  flow  out,  for  the 


PREPARATION    OF   OXYGEN.  11$ 

same  reason  as  that  which  prevented  water  from  flowing 
out  of  the  cylinders  which  we  filled  with  water  and 
inverted  under  water  in  the  pneumatic  trough.  For,  as 
the  vessel  A  is  closed  completely  at  the  top,  air  cannot 
get  into  that  vessel,  and  therefore  the  water  cannot 
flow  out  at  the  bottom.  But  if  we  now  lead  oxygen 
gas  into  A,  by  the  opening  D,  the  vessel  A — that  is  to 


Fig.  29.— Gasholder.  Fig.  30.— Filling  a  gasholder  with  oxygen  gas. 

say,  our  gasholder — will  get  filled  with  oxygen,  while 
water  will  flow  out  (at  D)  as  the  oxygen  passes  in. 

To  prepare  oxygen,  we  place  potassium  chlorate  in 
the  retort  M  (fig.  30),  and  heat  it ;  we  then  insert  the 
tube  N  into  the  opening  of  the  gasholder  D  ;  and  when 
the  vessel  is  filled  with  oxygen,  we  screw  the  cap  E 
on  to  D.  Were  we  now  to  open  the  stopcock  K,  water 
would,  of  course,  tend  to  flow  from  the  reservoir  into 


Il6      INTRODUCTION    TO    MODERN    CHEMISTRY. 

the  closed  lower  vessel ;  but  as  that  vessel  is  closed 
completely,  and  is  quite  filled  with  oxygen  gas,  the 
water,  not  being  able  to  enter  the  vessel,  would  exert 
a  pressure  on  the  gas  contained  therein.  But  if  we 
now  open  the  stopcock  F,  water  will  flow  into  A,  and 
the  gas  will  flow  out  at  H  in  proportion  to  the  inflow 
of  water  from  B  into  A.  As  the  entering  water  drives 
the  gas  before  it,  and  causes  the  gas  to  leave  the  gas- 
holder, we  can  lead  the  stream  of  gas  to  any  place 
where  we  wish  to  have  it  by  connecting  a  leading- tube 
to  H  by  caoutchouc  tubing ;  moreover,  we  can  store  the 
gas  in  the  gasholder  as  long  as  we  please. 

Oxygen  is  an  odourless  and  tasteless  gas.  It  is 
exceedingly  active — that  is,  very  disposed  to  enter  into 
combination.  Compounds  of  oxygen  are  known  with 
all  elements  except  fluorine  [argon,  helium,  and  the 
companions  of  argon].  The  compounds  which  are 
composed  of  oxygen  and  one  other  element  are  called 
oxides.  As  we  already  know  (see  p.  75),  the  bases  are 
oxides,  and  most  of  the  acids  are  formed  from  oxides. 
The  oxygen  compounds  of  the  non-metals  yield  acids  ; 
those  of  the  metals,  with  a  few  exceptions,  are  bases. 

If  we  now  burn  some  elements  in  pure  oxygen  gas, 
we  see  them  combining  therewith  with  great  brillancy. 
For  example,  let  us  place  a  little  phosphorus  on  a  small 
spoon  connected  to  a  long  wire  which  passes  through  a 
cork,  ignite  the  phosphorus,  and  plunge  it  into  a  jar  filled 
with  oxygen.  The  phosphorus  burns  with  a  brilliancy 
that  dazzles  the  eyes,  and  changes  into  white  clouds  (see 
fig.  31).  If  the  white  solid  that  is  formed  is  analysed 


ACIDS   AND   ACID   ANHYDRIDES. 


117 


quantitatively,  it  is  found  to  be  composed  of  43*66  per 
cent,  phosphorus  and  56-34  per  cent,  oxygen  ;  from 
which  result,  using  the  atomic  weights  [of  phosphorus 
(31)  and  oxygen  (16)],  it  follows  that  the  substance 
is  formed  of  two  atoms  of 
phosphorus  and  five  atoms 
of  oxygen  :  hence  the  formula 
of  the  compound  is  P2O-,.  It 
is  called  phosphorus  pentoxide 
(Greek  irevre  —five).  But  this 
compound  is  also  called  phos- 
phoric anhydride.  The  word 
anhydride,  derived  from  the 
Greek  v&op  (=  water),  signifies 
without  water.  The  second 
name  is  given  because  the  oxide 
produces  phosphoric  acid  as 
soon  as  it  comes  into  contact 
with  water  :  with  one  molecule 
of  water  the  oxide  produces  a  special  acid  called  meta- 
phosphoric  acid.  (We  shall  learn  hereafter  why  this 
acid  is  not  called  simply  phosphoric  acid.)  The  oxide 
itself  is  not  an  acid ;  it  is  an  acid  anhydride.  These 
two  conceptions,  acid  and  acid  anhydride,  must  not 
be  confused. 


Fig.  31  —Phosphorus  burning 
in  oxygen. 


PA 

One  molecule  phosphorus 
pentoxide 


H,0 

one  molecule 
water 


2HP03. 

two  molecules 

metaphosphoric  acid. 


Metaphosphoric  acid,  having  the  composition  HPO3,  is  called 
anhydrous  metaphosphoric  acid,  in  distinction  to  phosphoric 
anhydride.  This  compound  does  not  contain  any  water,  in  the 
sense  that  water  itself  is  present  in  the  compound  ;  but  it  is  only 


Il8      INTRODUCTION   TO   MODERN    CHEMISTRY. 

when  a  molecule  of  water  has  combined  chemically  with  the 
anhydride  that  the  anhydride  is  changed  to  an  acid.  If  this  acid, 
which  has  the  composition  expressed  by  the  formula  HPO;t,  is 
dissolved  in  water,  the  liquid  is  called  an  aqueous  solution  of 
metaphosphoric  acid ;  and  by  using  less  or  more  water,  a  con- 
centrated or  a  dilute  solution  is  obtained. 

Exactly  similar  conditions  hold  good  with  sulphuric  acid,  for 
example.  (We  shall  learn  about  the  preparation  of  this  acid  when 
we  come  to  consider  sulphur.)  The  oxide  of  sulphur,  SO.,,  is 
called  sulphur  trioxide,  or  sulphuric  anhydride,  just  as  we  speak 
of  phosphorus  pentoxide  or  phosphoric  anhydride.  Sulphuric 
acid,  H2SO4,  is  formed  by  combining  the  oxide  with  one  molecule 
of  water. 

SO3  +   H2O  =       H2S04. 

Sulphur  trioxide  +  water  =  sulphuric  acid. 

The  compound  H.,SO4  is  sulphuric  acid ;  but  for  the  chemist 
it  is  anhydrous  sulphuric  acid,  for  the  molecule  of  water  added 
to  SO.,  is  an  essential  part  of  sulphuric  acid.  Sulphur  trioxide 
itself  is  an  oxide,  not  an  acid.  The  molecule  of  water  has  com- 
bined chemically  with  the  sulphur  trioxide,  SO3;  it  has  not  dis- 
solved the  SO3  after  the  manner  of  a  solvent.  It  is  only  when 
more  water  is  added  than  is  shown  in  the  foregoing  equation 
that  the  ordinary  sulphuric  acid,  containing  water,  is  produced. 
Commercial  sulphuric  acid  contains  about  97  per  cent.  H.,SO4— 
that  is,  97  per  cent,  anhydrous  sulphuric  acid — and  3  per  cent, 
water.  It  is  so  troublesome  to  remove  this  last  3  per  cent,  of 
water  that  it  is  allowed  to  remain  in  the  commercial  acid. 

To  recapitulate.  Chemists  distinguish  between  acid  anhydrides, 
which  are  nothing  but  oxides,  and  anhydrous  acids  :  the  latter 
are  composed  of  the  oxides  with  one,  two,  three,  or  more 
molecules  of  water.  As  the  anhydrides  are  not  acids  until  they 
have  combined  with  definite  quantities  of  water,  this  water 
belongs  to  the  constitution  of  the  acids. 

If  we  bring  a  glimmering  piece  of  charcoal,  C,  into  a 
flask  containing  oxygen,  it  glows  brightly,  and  burns  to 
CO2  by  combining  with  two  atoms  of  oxygen.  The 
compound  CO2  is  a  gas,  commonly,  but  incorrectly, 


FORMATION   OF  OXIDES.  I IQ 

called  carbonic  acid.  This  gas  is  properly  called 
carbon  dioxide,  or  carbonic  anhydride,  in  accordance 
with  what  was  said  concerning  phosphoric  anhydride 
and  sulphuric  anhydride.  One  or  other  of  the  two 
names  just  mentioned  is  always  given  to  this  compound 
in  accurately  scientific  works. 

Certain  substances  which  we 
are  not  accustomed  to  see  burn- 
ing will  burn  in  pure  oxygen 
gas ;  for  instance,  glowing  iron 
brought  into  this  gas  combines 
therewith  and  throws  out  a 
shower  of  sparks  (see  fig.  32). 
The  iron  burns,  as  we  should 
expect,  to  an  oxide — to  an  iron 
oxide,  which  has  the  formula 
Fe3O4.  Similarly  zinc  burns  to 
zinc  oxide,  ZnO.  Such  oxides, 
being  oxides  of  metals,  are  bases  oxygen, 

(see  p.  75),  and  they  combine  with  acids  to  produce 
salts.  Zinc  oxide  and  sulphuric  acid,  for  instance, 
form  zinc  sulphate. 

The  reaction  of  sulphuric  acid  with  zinc  oxide  proceeds  as 
shown  in  the  following  equation  : — 

H2SO,         -4-       ZnO       =        ZnSO4        +   HA 
Sulphuric  acid  -f-  zinc  oxide  =  zinc  sulphate  •+•  water. 

Such  a  salt  as  zinc  sulphate  used  to  be  called  sulphate  of  oxide 
of  zinc.  If  we  look  closely  at  the  formula  of  the  salt,  we  see 
that  the  metal — in  this  case  zinc — has  taken  the  place  of  both  the 
atoms  of  hydrogen  in  the  sulphuric  acid.  We  have  already  found 
that  hydrogen  gas  is  given  off  when  diluted  sulphuric  acid  is 
poured  upon  zinc.  Hitherto  we  have  paid  heed  to  this  part  of 


120      INTRODUCTION    TO   MODERN   CHEMISTRY. 

the  reaction  only.  But  if  we  examine  the  solution  from  which 
the  hydrogen  escapes,  we  find  that  it  contains  zinc  sulphate  in 
place  of  sulphuric  acid.  Hence  the  zinc  has  taken  the  place  of 
the  hydrogen  which  it  has  driven  out  of  the  sulphuric  acid. 

H,SO4         +  Zn   =       ZnSO4       +  H2. 

Sulphuric  acid  +  zinc  =  zinc  sulphate  -f  hydrogen  gas. 

As  the  formulae  of  salts  may  be  regarded  as  if  the  metals  in 
them  have  replaced  the  hydrogen  of  acids,  and  as  this  view  is 
accepted,  the  names  of  salts  are  not  nowadays  derived  from  the 
names  of  the  metallic  oxides,  but  from  the  names  of  the  metals 
themselves.  In  modern  chemistry  one  speaks  of  zinc  sulphate, 
copper  sulphate,  lead  acetate,  sodium  carbonate,  and  so  on. 

Oxygen  is  that  constituent  of  the  atmosphere  which 
makes  possible  those  combustions  which  we  are  familiar 
with  in  ordinary  life.  The  following  is  the  average 
composition  of  atmospheric  air  : — * 

Nitrogen       77-41  per  cent. 

Oxygen         2077 

Moisture      0*85       ,, 

Carbon  dioxide       ...      0*03       ,, 
Argon  0-94       „ 

100-00 


The  minute  traces  of  the  gases  metargon,  krypton, 
neon,  and  xenon,  which  have  been  discovered  recently 
in  the  air  by  extraordinarily  delicate  methods  of 
examination,  are  included  in  the  "94  per  cent,  of  argon 
in  the  foregoing  statement. 

Oxygen  gas,  then,  forms  about  one-fifth  of  any 
volume  of  air  ;  it  is  diluted  with  about  four-fifths  of 

*  The  quantities  of  nitrogen  and  oxygen  in  air  vary  very 
slightly ;  the  quantity  of  carbon  dioxide  varies  more ;  and  the 
quantity  of  moisture  varies  much.  [T.R.] 


COMBUSTIONS   IN   OXYGEN.  121 

other  gases.  Hence  processes  of  combustion  proceed 
more  moderately  in  air  than  we  saw  them  proceeding 
in  pure  oxygen. 

We  are  now  in  a  position  to  follow  what  happened 
when  we  set  fire  to  a  piece  of  paper  and  allowed  it  to 
burn  in  the  air,  at  the  very  beginning  of  our  considera- 
tions, in  order  to  obtain  some  notion  of  chemical 
processes.  Paper  is  nearly  pure  cellulose.  That  com- 
pound is  composed  of  6  atoms  of  carbon,  10  atoms 
of  hydrogen,  and  5  atoms  of  oxygen  ;  its  formula  is 
C6H10O6.  When  it  is  burnt,  the  carbon  is  changed 
to  carbon  dioxide  (CO2),  and  the  hydrogen  to  water 
(H2O).  To  do  this,  17  atoms  of  oxygen  are  required — 
namely,  12  atoms  of  oxygen  to  change  6C  to  6CO2, 
and  5  atoms  of  oxygen  to  change  loH  to  5H2O. 
The  cellulose  (C6H]0O5)  already  contains  5  atoms  of 
oxygen;  the  remaining  12  atoms  of  oxygen  must  be 
obtained  from  the  air.  The  equation  which  represents 
the  changes  that  occur  when  the  paper  burns  will  be  as 
follows  : — 

C6H1005  +        120      =  6CO,         +  5H20. 

Cellulose  +       oxygen     =    carbon  dioxide    +   water, 
(paper)         (from  the  air) 

We  shall  suppose  that  the  piece  of  paper  we  burned 
weighed  2  grams  ;  let  us  calculate— and  this  we  can  do 
easily — the  weight  of  the  products  of  burning  this  piece 
of  paper.  From  the  formula  C6H10O5,  and  the  atomic 
weights  C  =  12,  H  =  I,  O  =  16,  we  find  that— 

C6          H10          05 
(12  x  6)  +  10  +  (5  x  1 6)  =  72  -f  10  +  80  =  162. 


122      INTRODUCTION    TO   MODERN   CHEMISTRY. 

And,  treating  the  products  of  combustion  in  the  same 
way,  we  see  that — 

C  Os  H2          O 

6  (12  +  [2  x  1 6])  +  5  ([i  x  2]  +  16) 

6CO2          5H.O 
=  (6  x  44)  +  (5  x  1 8) 

=  264  -f  90  =  354. 

As  162  parts  by  weight  of  cellulose  yield  354  parts 
by  weight  of  combustion-products,  the  proportion 

162  :  354  =  2  :  x;     or,  x  =  3^*  2  =  4'37 

tells  that  2  grams  of  paper  yield  4-37  grams  of  com- 
bustion-products. 

The  products  of  combustion  are  heavier  than  the 
substance  that  was  burnt.  This  is  self-evident  from 
the  description  that  has  been  given  of  the  process  of 
burning,  because  the  burning  substance  combines  with 
oxygen,  and  the  increase  of  weight  must  be  equal  to 
the  weight  of  the  oxygen  wherewith  it  has  combined. 

The  fact  that  the  products  of  a  combustion  weigh 
more  than  the  combustible  substance  may  be  demon- 
strated in  the  following  way.  A  cylinder  (A,  fig.  33) 
is  suspended  from  one  end  of  the  beam  of  a  balance. 
A  piece  of  wire-gauze  is  fixed  in  the  cylinder,  and 
pieces  of  caustic  soda  (NaOH)  sufficient  to  fill  the 
cylinder  are  placed  on  the  gauze.  A  small  candle  is 
placed  under  the  cylinder,  and  the  balance  is  brought 
into  equilibrium  by  placing  another  similar  cylinder 
and  candle  on  the  other  pan.  When  the  candle  under 
A  is  lighted,  the  caustic  soda,  which  is  a  base,  will 


BURNING   A   CANDLE   IN   AIR.  -    123 

combine  with  the  carbonic  anhydride,  which  is  pro- 
duced by  the  burning  substance  (and  is  drawn  into 
the  cylinder  as  into  a  chimney),  and  sodium  carbonate 
will  be  produced.  As  caustic  soda  is  one  of  those 
substances  which  absorb  water  (see  p.  45),  it  will  also 
retain  the  water  that  is  formed  by  burning  the  hydrogen 
of  the  wax.  These  two  substances,  carbon  dioxide 


Fig.  33.— Experiment  to  show  that  the  products  of  combustion  of  a  candle 
weigh  more  than  the  candle. 

and  water,  are  the  only  products  of  burning  wax,  as 
that  substance  is  composed  of  the  elements  carbon, 
hydrogen,  and  oxygen.  The  products  of  combustion, 
in  this  case,  must  weigh  more  than  the  candle,  for  the 
same  reason  as  held  good  in  the  case  of  the  paper ; 
therefore  we  notice  that  the  pan  whereon  the  burning 
candle  is  placed  sinks  soon  after  the  candle  is  lighted, 
and  we  thus  see  that  this  pan  of  the  balance  becomes 
heavier  as  the  candle  burns. 


124      INTRODUCTION    TO    MODERN    CHEMISTRY. 

The  reason  why  the  products  of  a  combustion  weigh 
more  than  the  combustible  substance  is  clear  from  the 
statements  and  descriptions  which  have  been  given. 
But  the  explanation  of  the  phenomena  of  combustion, 
which  we  have  been  able  to  grasp  so  easily  because 
of  our  knowledge  of  the  reactions  of  oxygen,  was 
formerly  beset  with  many  difficulties,  and  had  to  wait 
two  thousand  years  before  it  was  established.  We 
are  still  accustomed  to  speak  of  the  destruction  of 
substances  by  fire,  because  the  combustible  materials 
generally  disappear  from  sight  during  the  process  of 
burning.  And  in  the  olden  times,  when  no  clear 
conception  had  been  formed  of  gases,  because  they 
could  not  be  directly  seen,  the  opinion  prevailed  that 
the  ponderable  matter  was  not  only  apparently,  but 
really,  destroyed  by  burning.  There  was  supposed  to 
be  a  fire  substance  which  resolved  the  combustible 
body  into  non-existence. 

About  two  hundred  years  ago  people  began  to 
investigate  special  combustion-products,  which  they 
had  gradually  learnt  to  collect  (for  at  that  time  men 
were  not  accustomed  always  to  consult  the  balance  in 
chemical  investigations);  but,  still  hampered  by  the 
very  ancient  view — which  is  met  with  in  the  Bible — they 
came  to  a  conclusion  which  was  diametrically  opposed 
to  the  actual  facts.  They  adopted  the  opinion  that  the 
products  of  burning  were  present  in  the  combustible 
material  before  burning,  and  that  the  combustion-process 
consisted  in  setting  free  these  products  from  the 
combustible  substance.  Thus  regarded,  a  combustion 
process  was  a  decomposition  of  the  combustible  material 
into  the  products  of  combustion  ;  it  was  a  process  of 


PHLOGISTON.  125 

division,  whereas  in  fact  it  is  a  process  of  addition, 
and  consists  in  the  taking  up  of  oxygen  by  the  com- 
bustible material,  with  the  appearance  of  fire. 

In  the  seventeenth  century  it  was  supposed  that 
the  gas  which  is  given  off  when  charcoal  burns  (now 
called  carbonic  anhydride;  see  p.  119)  pre-existed  in 
the  charcoal.  (We  know  to-day  that  this  gas,  CO2,  is 
formed  by  the  union  of  carbon,  C,  with  oxygen,  O, 
in  the  air.)  Charcoal  (carbon)  was  supposed  to  be 
composed  of  this  gas  and  the  principle  of  fire.  In  the 
same  way  it  was  thought  that  the  brown  substance 
which  is  formed  by  heating  iron  in  the  air  already 
existed  in  the  iron.  (We  know  to-day  that  the  sub- 
stance is  an  oxide  of  iron — a  compound  of  iron  with  , 
the  oxygen  of  the  .air.)  Following  this  na'ive  way  of 
looking  at  the  matter,  metallic  iron  would  be  regarded 
as  composed  of  iron  oxide  and  a  principle  of  com- 
bustibility. 

This  combustible  principle  was  supposed  to  be  fixed 
in  the  metals,  a  view  which  certainly  seems  to  us 
very  odd.  Stahl,  who  was  a  very  able  chemist  in 
his  day,  at  the  beginning  of  the  eighteenth  century 
developed  this  view  into  a  general  theory,  which  was 
destined  to  rule  the  chemical  world  for  nearly  a  hundred 
years.  The  theory  declared  that  the  phenomena  of 
combustion,  be  the  combustible  bodies  what  they  may, 
depend  on  the  presence  of  a  something  which  is  common 
to  all  bodies  that  can  be  burnt :  if  this  something  is 
called  fire-stuff,  then  the  same  fire-stuff  is  in  all  com- 
bustible substances.  Stahl  called  this  thing  phlogiston, 
which  means  burnt  (to-day  it  would  be  translated 


126      INTRODUCTION    TO   MODERN    CHEMISTRY. 

combustible).  According  to  Stahl,  the  better  a  body 
burns,  the  richer  it  is  in  phlogiston  ;  hence  carbon  and 
phosphorus  contain  very  much  phlogiston.  It  followed 
that  combustion  consists  in  the  outrush  of  phlogiston 
from  the  burning  body.  It  must  be  confessed  that  this 
theory,  which  seems  to  us  so  remarkable,  explained 
most  of  the  phenomena  of  combustion  known  at  the 
time,  as  long  as  the  aid  of  the  balance  was  not 
invoked. 

If  phlogiston  escapes  during  combustion,  this  loss 
must  cause  the  product  of  combustion  to  be  lighter 
than  the  unburnt  material ;  but  no  one  troubled  about 
this  at  the  time  we  are  speaking  of.  We  have  already 
convinced  ourselves  of  the  accuracy  of  a  conclusion  just 
the  opposite  of  that  drawn  by  the  phlogistic  theory, 
both  by  calculating  the  weight  of  the  combustion 
products  of  paper  compared  with- that  of  the  unburnt 
substance,  and  also  by  the  ocular  demonstration  of  the 
burning  candle. 

We  have  already  seen  that  the  combustion-products 
are  heavier  than  the  combustible  material  which  yields 
them.  It  is  to  the  immortal  fame  of  Lavoisier  that  he, 
in  the  last  quarter  of  the  eighteenth  century,  con- 
clusively proved  the  inconsistency  of  the  phlogistic 
theory,  after  many  of  his  contemporaries  had  striven 
to  overthrow  it,  but  in  vain.  Lavoisier's  investigation, 
which  gave  the  finishing  stroke  to  the  whole  phlogistic 
theory,  is  markedly  simple,  like  many  another  experi- 
ment of  fundamental  importance.  He  also  was  the 
first  to  render  possible  the  building  of  modern  chemistry; 
for  it  was  by  his  investigations  that  a  final  explanation 


LAVOISIER'S  WORK  ON  COMBUSTION.       127 


was  given  of  the  processes  occurring  in  the  burning  fire, 
which  are  the  most  remarkable  and  striking  phenomena 
connected  with  material  changes  (and  therefore  are 
purely  chemical  phenomena)  that  are  met  with  without 
making  intentional  experiments.  All  chemistry  ante- 
cedent to  that  explanation  must  remain  piecework  in 
the  most  disparaging  sense  of 
the  term. 

For  the  purpose  of  his 
investigation,  Lavoisier  placed 
metallic  tin  in  a  retort,  and 
closed  the  end  thereof  by 
melting  the  glass  (see  fig.  34). 
Nothing  could  now  enter  the 
retort,  nor  could  anything 
escape  from  it.  Things  being 
thus  arranged,  he  -weighed 
the  retort  and  its  contents. 
The  retort  was  then  heated 
for  some  days  in  one  of  the  g 
charcoal  furnaces  that  were 
used  in  laboratories  at  that  Fig.  34.— Heating  tin "in'a  closed 

time    (represented    in    fig.     34).  retort  in  a  furnace. 

During  this  heating  the  tin  gradually  changed  to  what 
we  now  call  tin  oxide — that  is  to  say,  it  combined 
with  the  oxygen  of  the  air  in  the  retort.  At  that 
time,  when  oxygen  was  not  yet  known,  this  reaction  of 
the  metals  was  called  calcination  ;  the  tin  was  said  to 
be  changed  to  calx  of  tin.  A  weighing  of  the  retort 
and  its  calcined  contents,  after  cooling,  showed  that 
the  weight  was  the  same  as  before  heating;  hence  it 


128      INTRODUCTION    TO   MODERN    CHEMISTRY. 

followed  that,  contrary  to  the  requirement  of  the  theory, 
no  phlogiston  had  escaped  from  the  retort  during  the 
calcination  of  the  metal ;  for,  had  phlogiston  escaped, 
the  retort  and  its  contents  must  have  weighed  less  after 
the  calcination  of  the  tin  than  they  weighed  before. 
According  to  the  upholders  of  the  phlogistic  theory, 
phlogiston  was  so  light  and  thin  a  substance  that  it 
could  pass  through  glass  (somewhat  after  the  manner 
of  light) ;  hence  it  must  have  escaped  under  the 
conditions  of  the  experiment.  This  one  process  of 
weighing  made  by  Lavoisier  sufficed,  indeed,  to  disprove 
completely  the  phlogistic  theory. 

Proceeding  with  his  experiment,  Lavoisier  opened 
the  retort,  and  noticed  that,  thereupon,  air  rushed  into 
the  vessel.  On  weighing  once  more,  he  found,  of 
course,  that  the  whole  had  become  heavier.  He  then 
removed  the  calcined  tin  from  the  retort  and  weighed 
it.  He  found  that  the  increase  of  weight,  in  relation  to 
the  quantity  of  tin  used,  amounted  to  as  much  as  the 
retort  and  its  contents  had  gained  in  weight  after  air 
had  been  allowed  to  rush  in,  by  opening  the  retort, 
when  the  heating  in  the  furnace  was  finished.  The 
increase  in  the  weight  of  the  tin  was  just  equal  to  the 
weight  of  the  air  which  had  rushed  into  the  retort. 
Lavoisier  was  soon  afterwards  in  a  position  to  give  the 
correct  explanation  of  this  remarkable  fact. 

At  that  time— 1774 — oxygen  gas  had  just  been  dis- 
covered by  Priestley,  who  was  the  first  to  obtain  it  by 
the  method,  familiar  to  us,  of  heating  mercury  oxide. 
Lavoisier,  with  the  penetration  of  genius,  very  soon 
recognised  this  gas  (which  he  had,  meanwhile,  himself 
prepared)  to  be  the  constituent  of  the  air  which  brings 


LAVOISIER'S  WORK  ON  COMBUSTION.       129 

about  all  processes  of  combustion,  and  that  the  metallic 
calces  are  nothing  else  than  compounds  of  the  metals 
with  oxygen.  Moreover,  he  perceived  that,  notwith- 
standing the  long  duration  of  the  heating  process,  only 
a  portion  of  the  air  in  his  retort  had  combined  with  the 
metal ;  and  from  this  he  drew  the  conclusion  that  the 
air  must  be  composed  of  two  constituents  at  least. 

It  is  scarcely  possible  for  us,  to-day,  to  appreciate  the  revo- 
lution in  scientific  conceptions  that  was  caused  by  Lavoisier's 
elucidation  of  the  processes  that  occur  during  burning.  Until 
the  time  of  Lavoisier,  but  little  heed  was  given  to  the  quantita- 
tive prosecution  of  chemical  experiments,  the  importance  where- 
of has  already  been  sufficiently  brought  home  to  us.  It  was 
supposed  that  heat  was  something  ponderable,  although  no  one 
had  succeeded  in  establishing  its  weight.  All  results  of  weighing 
which  did  not  fit  in  with  the  older  views,  and,  consequently,  not 
with  the  phlogistic  theory,  were  attributed  to  changes  in  the 
weight  of  heat ;  and  all  those  quantitative  results  which  were  in 
contradiction  to  the  view  that  then  prevailed  were  attributed  to 
the  same  cause.  But  when  Lavoisier's  investigation  had  shown 
that,  without  doubt,  heat  played  no  part  in  combustions  so , far  as 
changes  of  weight  were  concerned — in  the  oxidation  of  tin  to  tin 
oxide,  for  example — but  that,  on  the  contrary,  heat  was  some- 
thing imponderable,  one  was  forced  to  acknowledge  that  all  other 
changes  of  weight  in  combustion-processes  were  dependent  on 
exchanges  of  material  substances — that  is,  were  purely  chemical 
occurrences. 

Lavoisier's  investigation  showed  that  the  atmosphere 
is  composed  of  at  least  two  kinds  of  airs — one  which 
supports  burning,  and  one  which  cannot  do  this.  It 
is  not  necessary,  of  course,  that  the  latter  should  be 
homogeneous ;  it  may  be  a  mixture  of  different  gases : 
we-already  know  that  the  former  is  oxygen  gas. 

The  following  experiment,  performed  with  a  little 

9 


I3O       INTRODUCTION   TO   MODERN   CHEMISTRY. 

piece  of  phosphorus,  will  convince  us  very  quickly 
that  oxygen  forms  only  a  small  part  of  atmospheric 
air.  As  a  preliminary,  let  us  burn  a  little  bit  of 
phosphorus  in  the  air.  The  burning  proceeds,  just  as 
it  does  in  pure  oxygen,  with  the  production  of  a  great 
deal  of  white  smoke  (see  A,  fig.  35).  Remembering 


Fig.  35.— Burning  phosphorus ;  and  determination  of  the  quantity  of  oxygen 
in  the  air. 

this,  we  now  place  a  little  bit  of  phosphorus  in  a 
small  basin,  and  attach  this  basin  to  a  cork,  which  is 
floated  on  water,  and  can  be  covered  by  a  bell-glass 
(see  B,  fig.  35).  Then  we  ignite  the  phosphorus 
floating  on  the  water ;  and  now  we  place  a  bell-glass 
over  the  burning  phosphorus  (as  shown  in  R,  fig.  35), 
which  continues  to  burn,  briskly  for  a  time,  in  the  air 
that  is  cut  off  by  the  water  from  the  surrounding 


BURNING  AND 

atmosphere.  We  see  white  clouds  of  phosphoric 
anhydride,  P2O5,  forming  in  the  bell-glass ;  but  the 
brilliancy  of  the  combustion  is  greatly  less  than  that  of 
phosphorus  burning  in  pure  oxygen.  The  phosphorus 
gradually  ceases  to  burn,  and  goes  out  long  before  all 
the  air  in  the  bell-glass  has  disappeared.  The  rising 
of  the  water  to  take  the  place  of  the  oxygen  in  the  bell- 
glass  (whose  disappearance,  or  rather  the  combination 
whereof  with  the  phosphorus  to  form  solid  phosphoric 
anhydride,  has  caused  a  vacuum  inside  the  glass)  shows 
us  that  about  one-fifth  of  the  air  has  vanished  (for  the 
water  now  fills  about  a  fifth  of  the  glass) ;  hence  this 
portion  of  the  air  must  have  combined  with  the  phos- 
phorus to  form  a  solid  body.  This  analysis  of  air 
shows  that  oxygen  forms  about  one-fifth  of  the  air 
that  surrounds  us. 

The  oxygen  of  the  air  is  used  up  in  all  processes 
of  burning ;  it  combines  with  everything  that  burns. 
This  occurs,  not  only  when  the  burning  is  accompanied 
by  the  appearance  of  fire  or  flame,  but  also  when  the 
burning  proceeds  without  that  purely  external  adjunct ; 
the  rusting  of  iron,  for  instance,  is  nothing  else  than 
the  union  of  iron  with  oxygen.  Processes  like  that 
just  mentioned  can  scarcely  be  described  appropriately 
as  burnings,  for  we  are  always  accustomed  to  connect 
burning  with  the  phenomena  of  fire  and  light.  The 
word  oxidation  is  used  in  such  cases :  chemists  speak 
of  the  rusting  of  iron  as  an  oxidation.  Although  oxida- 
tions may  occur  without  the  appearance  of  fire,  yet  they 
are  always  accompanied  by  the  production  of  the  heat 
which  can  be  made  available  by  the  combination  of  the 


132     INTRODUCTION   TO   MODERN    CHEMISTRY. 

substance  in  question  with  oxygen.  We  are  not  con- 
scious of  the  production  of  heat  in  such  a  process 
as  the  rusting  of  iron,  because  the  process  occupies 
so  long  a  time — it  may  be  spread  over  a  period  of  a 
year  or  more — that  we  do  not  notice  the  heat  that 
is  evolved. 

One  kilo,  of  carbon  produces  the  same  quantity  of 
heat  when  it  is  burned  very  rapidly  to  CO2  (see  p.  1 1 8) 
as  when  it  is  oxidised  quite  slowly  to  CO2  without  the 
appearance  of  fire  or  flame. 

Our  existence  depends  on  the  slow  oxidation  of 
carbon  to  which  we  have  referred,  in  so  far  as  the  heat 
of  our  bodies  is  maintained  by  such  a  gradual  oxidation 
of  substances  that  contain  carbon.  All  the  food-stuffs 
we  consume  contain  carbon.  We  notice  that  such 
substances  become  black  when  they  are  burned  ;  the 
expression  to  char,  which  is  in  common  use,  is,  there- 
fore, quite  correct.  When  the  processes  of  digestion 
have  acted  on  those  constituents  of  food-stuffs  that  are 
needed  for  our  nourishment  in  an  appropriate  way — 
that  is  to  say,  have  made  them  soluble  in  water — these 
constituents  reach  the  blood-stream,  in  forms  very 
different  from  their  original  states,  and  are  carried  by 
the  blood  to  those  parts  of  the  body  where  they  are 
required.  The  blood  passes  through  the  lungs  in  the 
course  of  its  circulation ;  and  in  these  organs  it  reaches 
certain  extremely  fine  veins,  wherein  it  comes  into 
contact  with  atmospheric  oxygen  that  has  diffused 
through  the  walls  of  these  veins.  The  blood  is  thus 
constantly  brought  into  contact  with  oxygen,  and  the 
oxygen  exerts  an  oxidising  action  on  those  compounds 


CARBON   DIOXIDE   IN   THE   BREATH.  133 

of  carbon  that  have  been  derived  from  the  food-stuffs 
taken  into  the  body.  The  blood  also  carries  with  it 
the  carbonic  anhydride,  CO2,  which  is  formed  in  the 
tissues,  and  this  gas  is  exchanged  for  oxygen  in  the 
lungs.  Hence  it  follows  that  expired  breath  is  very 
rich  in  carbonic  anhydride. 

The    presence    of    carbonic    anhydride    in   expired 
breath    may    be    demonstrated   . 
in   the   following    way.      First 
of   all  we   prepare  some  lime- 
water.     We    slake   some  burnt 
lime  (lime  has  the  composition 
CaO ;    it  is   the    oxide   of  the 
metal  calcium,  Ca)  by  pouring 
water    on    to    it.       The    lime 
combines      with      the      water 

With    the     production     of     much     FiS;  36.-Detection  of  carbon 

dioxide  in  expired  breath. 

heat  : — 

CaO       +    H20  =    Ca(OH)2. 
Burnt  lime  +  water  =  slaked  lime. 

If  much  water  is  added  to  our  slaked  lime,  a  solution 
is  produced  which  is  called  lime-water. 

We  now  place  some  clear  lime-water  in  a  flask  fitted 
with  a  cork  and  two  tubes,  as  shown  in  fig.  36,  and  we 
draw  a  stream  of  air  through  the  lime-water,  by  sucking 
at  A.  The  lime-water  remains  practically  unchanged  by 
the  air  that  passes  through  it.  The  quantity  of  carbon 
dioxide  contained  in  ordinary  air  is  so  small  (see  the 
analysis  of  air,  p.  120)  that  the  stream|of  air  would 
require  to  be  continued  for  a  long  time  to  make  the 
presence  of  the  carbon  dioxide  apparent.  £ut  matters 


134     INTRODUCTION   TO   MODERN   CHEMISTRY. 

are  very  different  when  we  send  our  expired  breath 
through  the  lime-water,  by  blowing  into  the  tube  G 
(fig.  36).  The  lime-water  becomes  turbid  in  a  very 
short  time,  through  the  formation  in  it  of  carbonate  of 
lime,  which  is  formed  by  the  union  of  the  lime  with  the 
carbonic  acid  in  the  breath ;  for  this  compound,  being 
insoluble  in  water,  soon  separates  as  a  solid,  and  we 
see  it  floating  in  the  water  in  the  form  of  a  white 
powder. 

The  following  equation  expresses  the  formation  of 
the  carbonate  of  lime  : — 

CO2  +  Ca  (OH)2  =          CaCO3        +  H2O. 

Carbonic  anhydride  +   slaked  lime   =  calcium  carbonate  +  water. 

We  have  used  the  name  calcium  carbonate  in  the 
equation  because  this  is  more  correct  than  carbonate 
of  lime  if  we  desire  to  follow  the  usual  notation  of 
salts  (see  p.  120). 

A  litre  of  air  weighs  1*293  grams  ;  hence  air  is  773 
times  lighter  than  water;  but  it  is  14*446  times  heavier 
than  hydrogen  gas.  As  a  column  of  mercury  760 
millimetres  [about  30  inches]  long  balances  the  weight 
of  the  air  at  the  sea-level — such  a  column  of  mercury 
is  used,  under  the  name  of  a  barometer,  for  measuring 
the  pressure  of  the  air  and  observing  the  oscillations 
of  that  pressure — we  are  able  to  calculate  the  weight 
of  the  atmosphere  that  surrounds  the  earth,  in  the 
following  way.  Taking  the  mean  pressure  of  the  air  on 
the  earth  as  equal  to  that  of  750  millimetres  of  mercury 
(because  of  the  many  mountains),  it  follows  that  a 
spherical  shell  of  mercury  the  size  of  the  earth's 


ACTION   OF   PLANTS  ON   CARBON   DIOXIDE.     135 

surface  and  750  millimetres  thick  would  have  the 
same  weight  as  the  sea  of  air  which  surrounds  the  earth- 
The  result  of  this  calculation  is  that  the  atmosphere 
weighs  5 -2  trillion  kilos,  and  contains  1*196  trillion  kilos, 
of  oxygen  [about  fifty  thousand  billion  tons,  containing 
about  eleven  thousand  billion  tons  of  oxygen]. 

The  mass  of  oxygen  in  the  atmosphere  is  so 
enormous  that  the  quantities  of  this  gas  used  for 
combustions,  and  for  the  breathing  of  men  and  animals, 
appear  unimportant.  Nevertheless,  nature  has  taken 
care  to  make  amends  for  what  is  used  ;  for  the  plants 
breathe  out  oxygen,  and  so  supply  that  gas  to  the 
atmosphere.  Plants  require  carbon  for  their  life-pro- 
cesses and  growth.  They  take  this  carbon  from  the 
air,  for  their  leaves  are  able  to  decompose  the  carbonic 
acid  (CO2)  in  the  air.  The  plants  retain  the  carbon 
they  require,  and  return  to  the  atmosphere  part  of  the 
oxygen  that  was  combined  with  carbon  in  the  carbonic 
acid  they  absorbed.  It  is  supposed  that  the  leaves 
exert  on  the  carbonic  acid  (CO2)  a  reducing  action — 
this  expression  here  signifies  adding  hydrogen — and 
that  the  first  product  of  this  action  is  a  compound 
called  formic  aldehyde^  which  has  the  composition 
COH2.  The  formation  of  this  compound  from  carbonic 
acid  may  be  represented  thus  : — 

CO2  +      H2      =        COH2       +      O. 

Carbonic  anhydride  +  hydrogen  =  formic  aldehyde  +  oxygen. 

Formic  aldehyde  is  extraordinarily  ready  to  enter 
into  chemical  reactions ;  it  is  a  body  which  yields 
numberless  complicated  compounds  with  great  readi- 
ness. Hence  the  chemist  easily  comprehends  that  it 


136     INTRODUCTION   TO   MODERN   CHEMISTRY. 

may  serve   for   the  building  up  of  the  most  various 
substances  in  living  plants. 

With  regard  to  the  presence  of  argon,  metargon,  krypton,  neon, 
and  xenon  in  the  atmosphere,  it  is  to  be  noted  that  these  gases 
(which  have  been  discovered  recently)  show  absolutely  no 
inclination  to  combine  with  other  elements :  not  a  single  com- 
pound of  any  one  of  them  is  knowp.  In  this  respect  they  are  the 
complete  opposite  of  all  the  elements  which  were  known  before 
their  discovery,  whose  numberless  compounds  form  our  earth. 
One  might  almost  say  that  these  five  elements  belong  to  another 
sort  of  world  than  ours. 

The  air  is  not  a  chemical  compound  of  certain  gases; 
it  is  a  mixture  of  these  gases.  The  gases  exist  in 
the  air  side  by  side,  the  one  uninfluenced  by  the  others. 
That  this  is  so  is  shown,  in  an  inverse  way,  by  the  fact 
that  if  oxygen  and  nitrogen  are  prepared  separately, 
and  are  mixed,  a  mixture  is  obtained  wherein  the  two 
gases  exhibit  their  properties  unchanged,  although  that 
mixture  behaves  exactly  like  air,  provided  the  propor- 
tion of  the  two  gases  is  the  same  as  in  air.  Moreover, 
when  air  is  shaken  with  water,  more  oxygen  dis- 
solves than  nitrogen,  which  would  not  be  probable  if 
the  two  gases  were  chemically  combined. 

We  now  pass  to  the  most  important  compound  of 
oxygen  with  hydrogen,  which  compound  is  water. 
We  know  that  when  a  flame  is  brought  to  a  mixture 
of  oxygen  gas  and  hydrogen  gas  these  gases  combine, 
with  a  loud  explosion,  to  produce  water  (see  p.  38).  If 
we  arrange  matters  so  that  the  two  gases  are  not  allowed 
to  mingle  until  the  moment  before  the  mixture  is 
lighted,  we  obtain  an  exceedingly  hot  flame — the  oxy- 


OXY-HYDROGEN    FLAME.  137 

hydrogen  flame.  We  do  this  in  the  following  way. 
Hydrogen  gas  is  led  through  the  tube  B  (fig.  37), 
and  is  ignited  at  A  :  we  now  have  a  hydrogen  flame 
which  must  get  its  oxygen  from  the  surrounding  air. 
Oxygen  is  now  led,  by  the  tube  c,  into  the  midst  of 
this  hydrogen  flame.  In  this  arrangement  the  hydro- 
gen and  oxygen  combine  in  the  flame  quite  quietly 
and  with  complete  safety.  The  oxy-hydrogen  flame 
finds  several  technical  applications — for  instance,  in 


37-  —  Burner  for  oxy-hydrogen  flame. 


manufacturing  platinum,  because  that  metal  cannot  be 
melted  in  any  ordinary  furnace.  If  this  extremely  hot 
flame  is  caused  to  impinge  on  an  infusible  substance 
—  on  lime,  for  instance  —  the  substance  (in  this  case  the 
lime)  is  raised  to  a  fervid  heat,  and,  consequently, 
emits  very  white  light.  This  arrangement  is  called  the 
limelight  :  it  has  been  used  for  many  purposes  —  for 
example,  in  lighthouses  ;  but  since  the  electric  arc- 
light  has  been  made  so  easily  accessible  the  limelight 
has  lost  its  importance. 

In  examining  the  methods  of  preparing  hydrogen 
we  found  that  water  is  decomposed  by  electrolysis  into 
its  two  components,  hydrogen  and  oxygen  (see  p.  33), 
and  thereby  we  made  a  qualitative  analysis  of  water. 


138     INTRODUCTION   TO   MODERN   CHEMISTRY. 

We  will  now  go  a  step  forward,  and  effect  a  quanti- 
tative determination  of  the  composition  of  water. 
We  shall  now  find  out  exactly  how  the  knowledge 
is  gained  that  water  consists  of  im  per  cent,  of 
hydrogen  and  88-89  per  cent,  of  oxygen,  with  the  help 
of  which  facts,  taken  as  known,  we  have  already  cal- 
culated the  formula  H2O  (p.  91).  Our  method  of 
procedure  depends  on  the  fact,  already  known  to  us, 
that  hydrogen  very  readily  combines  with  oxygen  to 
form  water.  This  is  the  reason  why  hydrogen  reduces 
most  oxides  of  metals,  at  a  high  temperature,  to  metals ; 
that  is  to  say,  hydrogen  removes  oxygen  from  such 
oxides  (combining  therewith  to  produce  water),  so  that 
the  metals  themselves  remain.  Copper  oxide,  CuO, 
is  a  metallic  oxide;  hence  metallic  copper  and  water 
are  obtained  when  hydrogen  gas  is  passed  over  red- 
hot  copper  oxide.  The  following  method  of  analysing 
water  quantitatively  is  made  possible  by  carrying  out 
this  process  with  the  helping  hand  of  the  balance  : — 

CuO         +          H2          =    Cu     +  H2O. 
Copper  oxide  +  hydrogen  gas  =  copper  +  water. 

Let  us  place,  not  any  indefinite  quantity,  but  an 
exactly  weighed  amount,  of  copper  oxide — say,  1*5634 
grams — in  the  bulb  A  of  the  apparatus  represented  in 
fig.  38,  and  let  us  pass  dry  hydrogen  gas  over  this 
copper  oxide.  We  obtain  the  hydrogen  from  a  Kipp's 
apparatus  (D,  fig.  38;  compare  p.  35);  but,  to  prevent 
the  gas  carrying  any  moisture  with  it,  we  dry  it  by 
leading  it  through  the  tube  G  before  allowing  it  to 
pass  over  the  copper  oxide.  The  drying-tube  G 
contains  a  salt,  called  calcium  chloride,  which  is  so 


QUANTITATIVE   SYNTHESIS   OF   WATER.        139 

hygroscopic — that  is,  eagerly  attracts  moisture — that 
gases  are  dried  completely  by  passing  over  it.  Of 
course,  we  might  have  dried  our  hydrogen  by  means  of 
a  washing-flask  containing  sulphuric  acid,  but  we  wish 
in  this  case  to  exemplify  another  method  of  drying. 

A  stream  of  dry  hydrogen  gas  is  now  passing  over 
the  copper  oxide  in  the  bulb  A  of  our  apparatus.  If 
we  now  heat  the  bulb  and  its  contents,  the  copper 


Fig.  38.— Quantitative  synthesis  of  water. 

oxide  will  be  changed  to  copper  gradually  and  com- 
pletely. As  the  copper  oxide  is  black,  and  it  is  changed 
to  copper,  which  is  red,  we  can  easily  follow  the  progress 
of  the  reaction.  All  the  oxygen  of  the  oxide  will  be 
combined  with  hydrogen  at  the  end  of  the  process,  so 
forming  water.  The  water  that  is  thus  produced  is 
carried  onwards  by  the  stream  of  dry  hydrogen  that 
continues  to  flow  from  the  Kipp's  apparatus,  and 
reaches  the  tube  B  (fig.  38),  which  also  is  filled  with 
calcium  chloride.  This  tube  will  hold  fast  all  the 
water  which  is  produced  by  the  reduction  of  the  copper 
oxide  by  hydrogen  and  is  carried  forwards,  from  the 


140     INTRODUCTION    TO   MODERN    CHEMISTRY. 

bulb  A,  by  the  stream  of  dry  hydrogen.  The  tube  B 
must  have  been  weighed  before  the  experiment  began, 
and  must  be  weighed  again  when  all  the  copper  oxide 
has  been  changed  to  copper.  The  increase  in  the 
weight  of  this  tube  gives  the  weight  of  the  water  that 
has  been  produced  in  our  experiment.  We  find  this 
increase  of  weight  to  be  0*3535  gram. 

If  we  also  weigh  the  copper  in  the  bulb  A,  we  find 
that  its  weight  is  1-2491  grams;  the  loss  of  weight 
suffered  by  the  copper  oxide  is  therefore  0-3143  gram 
[1-5634  —  1-2491].  This  number,  -3143,  is  the  weight 
of  the  oxygen  which  was  combined  with  copper — 
forming  copper  oxide,  CuO — and  is  now  combined  with 
hydrogen,  forming  the  -3535  gram  of  water  which  we 
have  caught  in  the  calcium  chloride  tube  B. 

These  numbers  tell  us  that  '3535  gram  water  contains 
•3143  gram  oxygen  (because  this  is  the  quantity  of 
oxygen  that  was  contained  in  the  1*5634  grams  copper 
oxide  which  was  reduced  to  copper).  If  we  deduct  this 
number,  -3143,  from  the  weight  of  water  formed — that 
is,  from  "3535 — the  difference  must  be  the  weight  of 
hydrogen  which  has  combined  with  the  '3143  gram  of 
oxygen.  The  weight  of  hydrogen  is  ['3535  —  '3143] 
•0392  gram ;  hence  our  quantitative  experiment  tells 
us  that  -3535  gram  water  consists  of  -3143  gram 
oxygen  and  '0392  gram  hydrogen.  We  have  only  to 
state  these  numbers  in  percentages  in  order  to  have 
the  composition  of  water  expressed  in  the  form  we  are 
accustomed  to  employ.  To  do  this  we  make  use  of 
the  two  proportions  : — 

(i)  -3535  gram  water  :  '0392  gram  hydrogen  =  100  :  x  ; 
(")  "3535  gram  water  1-3143  g^m  oxygen      =;  100  :  y  \ 


HYDROGEN   PEROXIDE.  141 

whence  we  find  that  x  =  ii'ii  and/  =  88*89.  The 
quantitative  synthesis  of  water,  then,  shows  that  com- 
pound to  be  composed  of  IITI  per  cent,  of  hydrogen 
and  88*89  per  cent,  of  oxygen. 

Another  compound  of  hydrogen  and  oxygen  is 
known  besides  water;  it  is  called  hydrogen  peroxide. 
That  compound  is  richer  in  oxygen  than  water  by  one 
atom ;  H2O  +  O  =  H2O2.  It  contains  more  oxygen 
than  any  other  known  compound.  The  molecular 
weight  of  the  compound  is  34  (H2  =  2  +  O2  =  32)  ; 
hence  34  parts  by  weight  contain  32  parts  by  weight, 
corresponding  to  94' I  per  cent,  of  oxygen.  We  shall 
consider  the  preparation  of  hydrogen  peroxide  under 
sodium. 

There  is  a  modification  of  oxygen,  called  ozone.  But 
we  cannot  properly  understand  how  a  substance  which 
is  an  element  (or,  as  one  might  say,  a  thing  in  itself) — 
and  oxygen  is  an  element — can  exist  in  several  forms,  in 
several  modifications  (for  that  seems  to  be  impossible), 
until  we  have  grasped  clearly  the  conception  of  the 
valency  of  atoms.  But  that  conception  cannot  be 
elucidated  until  we  have  acquired  more  purely  chemical 
knowledge  than  we  possess  at  present.  We  shall 
therefore  proceed  to  collect  such  knowledge. 


WE  know  that  fluorine,  chlorine,  bromine,  and  iodine 
form  a  group  of  four  elements  which  are  very  much 
alike  in  their  chemical  relations.  In  like  manner 
oxygen  belongs  to  a  group  of  four  elements  which 
are  chemically  very  similar.  These  four  elements  are 
oxygen,  sulphur,  selenion,  and  tellurium. 


SULPHUR. 

Sulphur  is  found  in  abundance  on  the  earth's  surface. 
Sulphur  itself — native  sulphur,  as  it  is  called — occurs 
in  volcanic  districts ;  in  Sicily,  for  example,  which 
supplies  the  requirements  of  the  whole  of  Europe. 
But  much  larger  quantities  of  sulphur  are  found  in 
combination  with  metals,  in  all  parts  of  the  world; 
and  very  considerable  quantities  of  sulphur  combined 
with  oxygen  and  a  metal  are  found  in  the  form  of 
gypsum,  which  is  calcium  sulphate. 

Crude  sulphur  is  worked  in  the  Sicilian  mines.  It 
is  purified  by  distillation,  whereby  the  impurities  (such 
as  mineral  debris)  that  adhere  to  it  are  left  behind  in 
the  distilling  vessel.  If  the  distillation  is  conducted 
rapidly,  the  chambers,  into  which  quantities  of  sulphur- 
vapour  are  poured,  become  so  hot  that  the  sulphur 

142 


DISTILLATION  OF   SULPHUR. 


143 


melts  in  these  chambers ;  when  it  has  cooled  and  has 
been  removed  from  the  chambers,  this  forms  the  stick- 
sulphur  of  commerce.  If  the  distillation  is  allowed  to 
proceed  slowly,  the  sulphur-vapour  falls  down  like 
rain  in  the  form  of  a  fine  dust,  and  does  not  melt 


Fig.  39. — Purifying  sulphur  by  distillation. 

in  the  comparatively  cold  chambers.  This  dust  is 
the  commercial  flowers  of  sulphur.  The  ease  where- 
with sulphur  can  be  distilled  permits  the  exhibition  of 
the  process  in  such  an  apparatus  as  is  represented  in 
fig.  39.  The  sulphur  is  heated  in  the  retort  A,  and 
distils  into  the  large  glass  balloon  B,  which  represents 
the  brick-built  chambers  of  the  factory. 


144     INTRODUCTION   TO   MODERN   CHEMISTRY. 

Sulphur  combines  with  almost  all  other  elements, 
just  as  oxygen  does.  The  formation  of  iron  sulphide, 
from  iron  and  sulphur,  was  one  of  the  first  chemical 
experiments  we  performed.  The  first  compound  of 
sulphur  we  shall  now  consider  is  that  with  hydrogen. 
This  compound  is  a  gas,  and  is  called  sulphuretted 
hydrogen.  Hydrogen  is  obtained  when  a  acid — say 
hydrochloric  acid — reacts  with  iron;  if  iron  sulphide 
is  substituted  for  iron,  the  gas  which  is  formed  is 
sulphuretted  hydrogen.  The  following  equations  give 
a  clear  presentment  of  these  reactions  : — 

(i)  2HC1  +  Fe  =          H2          +       FeCl2. 

Hydrochloric  acid  +  iron  =  hydrogen  gas  +  iron  chloride. 

(ii)  2HC1      +         FeS        =  H2S  +       FeCl2. 

Hydrochloric  +  iron  sulphide  =  hydrogen  sulphide  +  iron  chloride 
acid  gas 

Sulphuretted  hydrogen  is  a  colourless  gas ;  it  is  at 
once  recognised  by  its  very  disagreeable  odour  of 
rotten  eggs.  As  one  might  suppose,  it  is  more  correct 
to  say  that  rotten  eggs  smell  of  sulphuretted  hydrogen  : 
the  albumin  of  eggs  contains  sulphur,  and  when  the 
eggs  decay  the  albumin  is  completely  decomposed 
and  sulphuretted  hydrogen  gas  is  given  off.  Sul- 
phuretted hydrogen  is  very  poisonous.  The  gas  may 
be  burnt ;  the  products  of  its  combustion  are  water 
and  sulphur  dioxide,  for  the  hydrogen  changes  to 
water,  and  the  sulphur  combines  with  two  atoms  of 
oxygen,  just  as  carbon  does  when  it  is  burnt.  As 
carbon  gives  carbon  dioxide,  so  sulphur  produces  sul- 
phur dioxide ;  and  as  carbon  dioxide  is  also  carbonic 


PROPERTIES   OF   SULPHURETTED   HYDROGEN.    145 

anhydride,    so     sulphur    dioxide    is    also   sulphurous 
anhydride. 

H2S  +     30     =  H.,O  +  SO2. 

Sulphuretted  hydrogen  +  oxygen  =  Water  +  sulphurous  anhydride, 
(from  the  air) 

Sulphuretted  hydrogen  is  only  slightly  soluble  in 
water  :  it  dissolves  to  the  extent  of  about  half  a  per 
cent. ;  that  is  to  say,  one  litre  of  water  [1000  grams] 
saturated  with  sulphuretted  hydrogen,  by  passing  the 
gas  into  the  water  until  no  more  gas  dissolves,  contains 
about  5  grams  of  the  compound.  We  found  that  one 
litre  of  water  dissolves  about  400  grams  of  hydrochloric 
acid  gas  (p.  72). 

In  spite  of  its  smell,  which  is  certainly  not  agreeable, 
sulphuretted  hydrogen  is  much  used  in  chemical 
laboratories ;  for  it  is  practically  indispensable  for 
the  detection  of  the  metals,  notwithstanding  the  many 
endeavours  that  have  been  made  to  find  a  less  evil- 
smelling  substitute. 

We  will  endeavour  to  set  forth  shortly  the  reasons 
for  the  usefulness  of  sulphuretted  hydrogen  in  analysis, 
and  at  the  same  time  to  give  an  approximate  present- 
ment of  the  methods  generally  followed  in  analysis. 
One  cannot  learn  analysis  properly  by  demonstrations, 
much  less  by  descriptions ;  the  only  way  is  personal 
work  in  the  laboratory. 

Almost  all  analyses  are  made  in  the  wet  way,  that 
is  to  say,  the  substance  to  be  analysed  is  brought  into 
solution.  To  do  this  presents  no  great  difficulties  in 
the  case  of  metals,  for  most  of  these  are  dissolved 

10 


146     INTRODUCTION    TO    MODERN    CHEMISTRY. 

by  hydrochloric  acid,  nitric  acid,  or  aqua  regta  (which 
we  shall  soon  become  acquainted  with).  Even  such 
hard  and  solid  bodies  as  granite  and  the  like  may  be 
transformed  into  compounds  suitable  for  analytical 
purposes,  and  soluble  even  in  water,  without  very  great 
difficulty.  For  this  purpose  the  finely  powdered  rock 
is  mixed  with  a  strong  alkali,  and  the  mixture  is  fused 
in  a  crucible.  The  silicic  acid  of  the  rock  becomes 
alkali  silicate,  which  is  soluble  in  water  (see  p.  74) ; 
when  this  has  been  dissolved  in  water,  the  residue 
consists  for  the  most  part  of  the  bases — such  as  lime, 
magnesia,  iron  oxide,  etc. — which  were  combined  with 
silicic  acid  in  the  original  substance,  and  these  bases 
are  soluble  in  various  acids,  in  hydrochloric  acid,  for 
example. 

When  sulphuretted  hydrogen  is  passed  into  a  liquid 
wherein  certain  metals  are  dissolved,  it  precipitates  the 
metals  as  sulphides;  that  is  to  say,  the  sulphuretted 
hydrogen  converts  the  metals  into  sulphur  compounds 
which  are  insoluble  in  water,  and,  therefore,  must 
separate  from  the  solution.  We  have  once  already 
made  use  of  a  solution  of  silver.  (We  shall  become 
acquainted  with  the  preparation  of  such  a  solution 
when  we  study  nitric  acid.)  If  we  pass  sulphuretted 
hydrogen  into  this  solution,  it  becomes  brownish  black, 
for  the  silver  is  changed  to  silver  sulphide,  which  has 
that  colour. 

The  apparatus  represented  in  fig.  40  may  be  used. 
The  flask  A  contains  iron  sulphide  and  hydrochloric 
acid.  The  sulphuretted  hydrogen  which  is  formed  by  the 
interaction  of  these  two  substances  is  washed  in  B,  and 
then  passes  into  the  silver  solution  in  c.  This  apparatus 


USES  .OF   SULPHURETTED   HYDROGEN.          147 

must,  of  course,  be  placed  in  a  draught-cupboard  (see 
p.  41).  The  silver  sulphide  soon  settles  down,  as  a 
precipitate,  in  c  ;  and  it  may  be  collected  on  a  filter.  To 
remove  the  silver  sulphide  from  the  liquid  we  proceed 
just  as  we  did  when  we  separated  sand  from  a  boiling 
solution  of  benzoic  acid  (see  p.  54).  The  filtrate — that  is, 
the  liquid  which  runs  through  the  filter — is  now  free 
from  silver,  and  can  be  examined  for  any  other  sub- 
stances which  it  may  contain.  In  conducting  such  an 


Fig.  40. — Leading  washed  sulphuretted  hydrogen  into  a  solution 
of  a  metal. 

examination  heed  must  be  given  to  the  following  facts' 
If  solutions  are  made  alkaline,  by  adding  ammonia — for 
ammonia  is  an  alkali  (compare  p.  75) — then  sulphuretted 
hydrogen  precipitates  all  the  heavy  metals,  as  sulphides, 
from  such  alkaline  solutions ;  but  if  the  solutions  are 
made  acid,  then  sulphuretted  hydrogen  precipitates  only 
some  of  these  metals. 

The  metals  in  a  solution  may  then  be  divided  into 
two  main  classes,  by  the  use  of  sulphuretted  hydrogen, 
in  the  following  manner.  Some  hydrochloric  acid  is 
added  to  the  solution,  and  the  gas  is  passed  into  this 


148     INTRODUCTION    TO   MODERN   CHEMISTRY. 

acid  liquid.  Certain  sulphides  are  now  precipitated — for 
example,  copper  sulphide,  lead  sulphide,  and  tin  sul- 
phide. The  liquid  is  filtered,  and  the  sulphides  of 
the  metals  of  the  first  class  are  obtained  on  the  filter. 
The  filtrate — which  is  now  free  from  copper,  lead, 
etc. — is  made  alkaline  by  the  addition  of  ammonia, 
and  sulphuretted  hydrogen  is  passed  into  it.  If 
a  precipitate  is  produced,  that  precipitate  consists  of 
those  metallic  sulphides  which  are  not  precipitated  from 
an  acid  solution,  such  as  iron  sulphide,  zinc  sulphide, 
etc.  The  second  class  of  metallic  sulphides  may  be 
collected  on  a  filter. 

Our  method  of  procedure  has  shown  us  how  the 
metals  in  any  solution  may  be  divided  into  two  main 
classes  by  converting  them  into  their  sulphides.  The 
further  separation  of  the  individual  members  of  each 
class  does  not  concern  us  here. 

We  must  now  regard  the  solution  we  have  been 
examining  as  having  been  freed  from  all  heavy  metals 
by  means  of  sulphuretted  hydrogen.  Of  the  better- 
known  elements  it  can  now  contain,  only  perhaps 
calcium,  magnesium,  potassium,  and  sodium.  The  de- 
tection of  these  is  proceeded  with  by  methods  similar 
to  that  we  have  already  used  ;  for  instance,  any 
calcium  that  may  be  present  is  precipitated  as  calcium 
carbonate,  which  may  be  removed  by  filtration ;  and 
the  filtrate  may  be  examined  for  magnesium,  potassium, 
.and  sodium  by  appropriate  methods,  which  are,  broadly, 
like  those  we  have  employed. 

Sulphur  combines  with  chlorine,  bromine,  and  iodine 
tp  form  sulphur  chloride,  sulphur  bromide,  and  sulphur 


OXY-ACIDS   OF   SULPHUR. 


149 


iodide ;  but  a  survey  of  these  compounds  would  not 
benefit  us  much.  The  compounds  of  sulphur  with  oxy- 
gen are  as  important  as  that  compound  with  hydrogen 
we  have  been  considering.  These  oxygen  compounds 
form  acids  by  reacting  with  water.  Of  those  acids 
which  are  thus  derived  from  sulphur  we  shall  make  the 
acquaintance  of  sulphurous  acid,  sulphuric  acid,  and 
thiosulphuric  acid,  passing  over  as  unimportant  to  us 
the  six  other  oxy-acids  of  sulphur.  No  other  element 
furnishes  so  many  acids  of  this 
kind  [that  is,  acids  containing 
oxygen]  as  sulphur. 

We  already  know  (p.  144)  that 
the  gaseous  oxide  SO2 — called 
sulphur  dioxide  or  sulphurous 
anhydride — is  formed  when  sul- 
phur burns  in  air  ;  the  same  oxide 
is  formed  when  sulphur  is  burnt 
in  oxygen.  Sulphur,  then,  like 
carbon,  disappears  when  it  is 
burnt ;  the  ordinary  expression  is 
that  "fire  destroys  sulphur  as  it 
destroys  carbon."  In  sulphurous  acid  gas  (which 
is  a  third  name  for  the  compound  also  called  sulphur 
dioxide  and  sulphurous  anhydride)  one  atom  of  sul- 
phur is  combined  with  two  atoms  of  oxygen.  If  we 
set  fire  to  a  little  sulphur  and  allow  it  to  burn  in  a 
flask  containing  some  water,  as  shown  in  fig.  41,  the 
sulphur  dioxide  which  is  produced  will  dissolve  in  the 
water,  as  it  is  a  gas  which  is  very  soluble  in  water.  If 
we  now  pour  some  litmus  solution  into  the  flask,  the 


Fig.  41.— Experiment  to 
show  that  an  acid  is 
formed  by  burning 
sulphur. 


I5O     INTRODUCTION   TO   MODERN    CHEMISTRY. 

blue  litmus  will  be  turned  red,  a  proof  that  the  solution 
in  water  of  the  gas  formed  by  burning  sulphur  contains 
an  acid. 

Sulphurous  acid  has  the  composition  shown  by  the 
formula  H2SO3  (SO2  +  H2O  =  H2SO3).  The  acid,  of 
course,  forms  salts.  The  sodium  salt  is  Na2SO3  (com- 
pare p.  120).  The  most  convenient  way  of  preparing 
this  salt  is  to  neutralise  the  acid  by  caustic  soda — 
that  is,  to  add  caustic  soda  to  a  solution  of  the  acid 
containing  a  little  litmus  until  the  red  colour  of  the 
litmus  just  begins  to  turn  blue.  (A  similar  method 
is  generally  the  most  convenient  for  preparing  other 
sodium  salts.)  The  chemical  reaction  is  expressed 
thus  in  an  equation : — 

H2SO3        +      2NaOH     =     Na2SO3  +  2H2O. 

Sulphurous  acid  +  caustic  soda  =  sodium  sulphite  +  water. 

We  have  seen  before,  we  see  here,  and  we  shall  always 
see,  that  water  is  formed,  besides  a  salt,  whenever  an 
acid  and  a  base  react.  The  accurate  description  of  acid 
and  base  is  that  they  are  compounds  which  by  their 
interaction  produce  a  salt  and  water  (compare  p.  74). 

Let  us  look  at  the  reaction  we  have  just  been  considering  in 
the  case  of  a  hydracid  of  one  of  the  halogens — say  hydrochloric 
acid.  When  this  acid  is  neutralised  by  caustic  soda  we  obtain 
common  salt  and  water  : — 

HC1  +       NaOH      =          NaCl  +    H2O. 

Hydrochloric  acid  +  caustic  soda  =  sodium  chloride  +  water. 

We  might,  therefore,  very  well  call  sodium  chloride  (NaCl) 
sodium  hydrochloride.  The  metal  sodium  takes  the  place  of  the 


SODIUM    THIOSULPHATE.  151 

atom  of  hydrogen  in  the  acid  HC1  (compare  p.  67).  In  NaCl 
the  element  chlorine  is  directly  joined  to  sodium,  forming  a 
salt.  Similar  relations  hold  good  in  the  cases  of  the  other 
three  halogens,  bromine,  iodine,  and  fluorine.  These  four  ele- 
ments combine  directly  with  metals  to  form  salts;  hence  the  name 
halogens  (salt  formers)  given  to  them  (from  the  Greek  &\s  —  salt). 
These  four  elements  are  characterised  by  this  reaction,  that 
they  form  salts  directly ;  whereas  not  the  other  elements  them- 
selves, but  their  oxides,  are  the  bodies  wherefrom  salts  are 
formed. 


If  sulphur  is  added  to  a  solution  in  water  of  sodium 
sulphite — which  we  have  spoken  of  a  moment  or  two 
ago — and  the  liquid  is  boiled  for  a  considerable  time, 
the  sulphur  gradually  dissolves,  although  sulphur  is 
quite  insoluble  in  water  alone.  As  the  solution  cools, 
?.  new  salt  crystallises  from  it,  in  place  of  the  sodium 
sulphite;  this  new  salt  is  sodium  thiosulphate.  [Com- 
mercially it  is  known  as  hyposulphite  of  soda  ;  and  it  is 
often  referred  to  merely  as  hypo7\ 

Na2SO3      +      S       =          Na2S2O3. 
Sodium  sulphite  +  sulphur  =-  sodium  thiosulphate. 

Sodium  thiosulphate,  Na2S2O3,  is  used  as  an  antichlor 
(see  p.  49).  As  soon  as  this  salt  is  added  to  a  solution 
of  bleaching  powder  the  two  salts  interact,  and  new 
substances  are  formed  which  do  not  injure  clothes  as 
bleaching  powder  does.  The  interaction  between  these 
two  compounds  is  very  complicated ;  we  shall  not, 
therefore,  write  down  the  equation  that  expresses  it,  as 
the  equation  is  not  easy  to  follow. 

Sodium    thiosulphate   is  also  much  used  in   photo- 


152     INTRODUCTION    TO   MODERN   CHEMISTRY. 

graphy.  In  order  to  obtain  a  picture,  plates  whereon 
is  spread  a  thin  film  of  silver  bromide  (compare  p.  60) 
are  exposed  in  the  camera.  The  rays  of  light  which 
proceed  from  the  object  to  be  pictured  decompose  the 
silver  bromide  exactly  in  proportion  to  their  strength. 
The  picture  which  is  formed  on  the  plate,  by  a  brief 
exposure  thereof,  and  is  not  visible  to  the  human  eye, 
is  strengthened  until  it  attains  the  desired  distinctness 
by  means  of  a  "  developer,"  in  the  dim  red  light  of  the 
dark-room.  Besides  the  picture,  all  the  silver  bromide 
which  has  not  been  affected  by  the  light  still  remains 
on  the  plate.  But  this  unchanged  silver  bromide  must 
be  removed  ;  for  if  the  plate  were  brought  into  the 
daylight,  the  unchanged  silver  bromide  on  it  would  be 
decomposed  by  the  light,  and  the  whole  plate  would  be 
blackened.  To  remove  the  unchanged  silver  bromide 
the  plate  is  laid  in  a  solution  of  sodium  thiosulphate 
after  the  development  of  the  picture.  Silver  bromide 
is  soluble  in  a  solution  of  sodium  thiosulphate,  whereas 
silver  bromide  which  has  been  changed  by  light — that 
is  to  say,  the  picture  formed  on  the  plate — is  insoluble 
therein.  The  silver  bromide  which  has  been  altered 
by  light  comes,  therefore,  unchanged  from  the  sodium 
thiosulphate  bath,  while  the  excess  of  silver  bromide — 
that  is,  the  portion  not  used  for  forming  the  picture — is 
removed. 

SULPHURIC  ACID. 

Sulphuric  acid  plays  an  incomparably  more  im- 
portant part  than  either  sulphurous  or  thiosulphuric 
acid.  The  formula  of  sulphuric  acid  is  H2SO4 ;  it 


SULPHURIC  ACID.  153 

contains,   therefore,   one   atom    of  oxygen    more    than 
sulphurous  acid. 

H2S03       +      O      =      H2S04.         , 
Sulphurous  acid  +  oxygen  =  sulphuric  acid. 

To  change  sulphurous  to  sulphuric  acid  it  is  only 
necessary  to  oxidise  the  first-named  acid — that  is,  to 
add  more  oxygen  to  it.  But  in  what  way  can  this  best 
be  done  ?  As  sulphuric  acid  is  used  in  enormous 
quantities  in  chemical  manufactures,  it  is  important 
that  the  oxygen  should  be  obtained  as  cheaply  as 
possible — that  is  to  say,  it  should  be  obtained  from  the 
air.  But  this  object  has  not  been  realised  without 
some  difficulty.  We  know  already  that  sulphur  burns 
in  the  air  only  to  sulphurous  anhydride,  SO2;  or,  to 
use  another  form  of  words,  that  by  mere  burning 
sulphur  is  oxidised  only  to  sulphur  dioxide.  Sulphur 
does  not  form  sulphuric  anhydride,  SO3,  by  direct 
combination  with  oxygen ;  in  other  words,  it  does  not 
burn  directly  to  sulphur  trioxide.  The  last-named 
oxide  would  certainly  at  once  form  sulphuric  acid  by 
reacting  with  water. 

SO,  +     O     =  S03. 

Sulphurous  anhydride  +  oxygen  =  sulphuric  anhydride. 

SO3  +  H2O  =      H2SOr 

Sulphuric  anhydride  +  water  =  sulphuric  acid. 

Tt  is  easy  enough  to  represent  these  changes,  on 
paper,  in  equations;  but  it  is  a  different  matter  to 
realise  them  in  practice. 

In  course  of  time  methods  have  been  found,  some- 


154     INTRODUCTION    TO    MODERN    CHEMISTRY. 

what  roundabout  it  is  true,  for  overcoming  all  the 
difficulties  in  the  way  of  carrying  out  the  desired 
oxidation.  As  sulphur  could  not  be  burnt  directly  to 
SO3,  an  instrument  was  sought  for,  and  has  been 
found,  which  should  carry  over  the  oxygen  of  the  air 
to  the  sulphurous  anhydride,  SO2,  which  is  produced 
by  the  direct  burning  of  sulphur.  This  process  serves 
to  oxidise  sulphur  to  the  oxide  SO3,  indirectly,  by 
means  of  the  oxygen  of  the  air.  Hence  it  attains  the 
wished-for  goal — namely,  the  preparation  of  sulphuric 
acid.  The  by-path  which  leads  to  this  result  is  the 
calling  in  the  help  of  nitric  acid  in  the  manufacture 
of  sulphuric  acid. 

Nitric  acid,  with  which  we  shall  soon  become 
acquainted  in  detail,  is  very  rich  in  oxygen.  It  is  an 
acid  that  contains  nitrogen  ;  its  formula  is  HNO3.  The 
abundance  of  oxygen  in  this  acid  is  the  only  thing 
about  it  which  interests  us  at  present.  Let  us  calculate 
the  quantity  of  oxygen.  As  the  atomic  weights  of 
hydrogen,  nitrogen,  and  oxygen  are  I,  14,  and  16 
respectively,  the  molecular  weight  of  nitric  acid  is— 

H      N          O3 

i  +  14  +  (3  x  16)  -  63. 

In  63  parts  by  weight  of  nitric  acid  are,  therefore,  con- 
tained 48  parts  by  weight  of  oxygen  ;  hence,  putting  x 
as  the  percentage  of  oxygen,  we  have  the  proportion — 

48  x  ioo 
63  :  48  =  ioo  :  x\   or,  x  —  - — ^ =  76-2. 

Nitric  acid,  then,  contains  76*2  per  cent,  of  oxygen. 
Hence  we  are  not  surprised  that  nitric  acid  should  be 


MANUFACTURE  OF   SULPHURIC   ACID.  155 

a  strong  oxidising  agent ;  or,  to  express  the  leading 
property  of  the  acid  without  making  use  of  this  technical 
term,  we  are  not  surprised  to  find  that  nitric  acid  is 
very  ready  to  give  up  oxygen  to  other  bodies. 

Sulphurous  anhydride,  SO2,  is  one  of  those  bodies 
whereto  nitrk  acid  readily  gives  up  part  of  its  oxygen. 
In  conducting  the  manufacture  of  sulphuric  acid 
matters  are  so  arranged  that  the  combustion  product 
of  sulphur — that  is,  gaseous  sulphurous  anhydride 
(SO2) — is  led,  along  with  steam,  into  chambers  made 
of  leaden  plates,  wherein  are  placed  pans  containing 
nitric  acid.  "Lead  chambers"  are  employed  because 
these  best  resist  the  action  of  sulphuric  acid.  The 
nitric  acid  exerts  an  oxidising  action  on  the  sulphurous 
anhydride  gas.  As  there  is  water  in  the  chambers  in 
the  form  of  steam,  the  wished-for  sulphuric  acid  is 
produced  from  the  materials  that  are  present,  and 
collects  as  a  liquid  at  the  bottom  of  the  chamber. 

SO2  +     H2O     +       O  H2SO4. 

Sulphurous  anhydride  gas  +    water  •    +   oxygen    ==a    sulphuric  acid, 
(obtained  by  direct  burn-        (in  the  form      (from  the 
ing  of  sulphur)  of  steam)       nitric  acid) 

As  the  oxygen  that  is  required  for  this  process  is 
obtained  from  nitric  acid,  that  acid  must  be  robbed  of 
part  of  its  oxygen.  Hence  one  would  suppose  that 
the  process  would  require  very  large  quantities  of  nitric 
acid.  But  the  requirement  is  only  apparent,  and  that  for 
the  following  reasons.  The  nitric  acid  certainly  gives 
the  oxygen  that  is  needed  for  the  formation  of  sulphuric 
acid  in  the  chambers;  but  the  compounds,  poorer  in 


156     INTRODUCTION    TO   MODERN   CHEMISTRY. 

oxygen  than  nitric  acid,  that  are  produced  by  this  giving 
up  of  oxygen  are  again  continuously  oxidised  to  nitric 
acid  by  the  oxygen  of  the  air,  which  is  intentionally 
allowed  to  flow  into  the  chambers  in  large  quantities, 
along  with  the  sulphur  dioxide  gas  and  the  steam.  It 
is  the  oxygen  of  the  air  which  keeps  constantly  going 
the  further  oxidation  of  the  sulphurous  anhydride,  by 
insuring  the  constant  presence  of  nitric  acid.  To  put 
it  shortly :  the  nitric  acid  acts  only  as  a  carrier  of  the 
oxygen  of  the  air  in  the  manufacture  of  sulphuric  acid.  A 
determinate  quantity  of  nitric  acid  should  suffice  for 
the  preparation  of  an  unlimited  amount  of  sulphuric 
acid ;  but  unavoidable  losses  in  the  manufacture  render 
it  necessary  to  make  very  small,  constant  additions  of 
nitric  acid.  Sulphur  is  not  an  expensive  material ; 
oxygen  from  the  air  is  cheap  enough ;  hence  the  low 
price  of  sulphuric  acid. 

The  sulphurous  anhydride  gas  required  in  the 
manufacture  of  sulphuric  acid  used  to  be  made  by 
burning  sulphur,  which  had  to  be  obtained  from  Sicily. 
But  cheaper  sources  have  been  made  use  of  for  the 
last  sixty  years.  Most  of  the  sulphur  used  in  Europe 
still  comes  from  Sicily;  but,  as  we  have  already 
remarked,  there  are  large  quantities  of  easily  worked 
sulphur  compounds — iron  sulphide,  for  instance — in 
various  other  places.  Native  iron  sulphide,  called 
pyrites  by  mineralogists,  has  the  composition  FeS2, 
and  contains  more  than  50  per  cent,  of  sulphur.  Like 
sulphur,  pyrites  may  be  burnt  in  a  kiln.  From  what 
we  have  learned  before,  we  can  see  that  the  products 
of  combustion  will  be  sulphurous  anhydride  gas,  SO2, 


USES   OF   SULPHURIC   ACID.  157 

and  iron  oxide,  Fe2O3 ;  and,  hence,  that  the  change  must 
be  expressed  by  the  following  equation  : — 

2FeS2         +          iiO        =         Fe2O3      ±  4SO2 

Pyrites  +  oxygen  =  iron  oxide  +  sulphurous  anhy- 
(iron  sulphide)  (from  the  air)  dride  gas. 

The  iron  oxide  which  remains  after  burning  pyrites  is  used 
in  iron-works.  The  possibility  of  making  use  of  this  by-product 
naturally  cheapens  the  price  of  sulphuric  acid. 

The  applications  of  sulphuric  acid  are  extraordinarily 
many.  This  acid  expels  most  other  acids  from  their 
compounds.  We  have  already  used  it  in  this  way  for 
making  hydrochloric  acid,  by  causing  it  to  react  with 
common  salt  (see  p.  67)  ;  it  is  also  employed  for  decom- 
posing phosphates,  as  we  shall  find  when  we  come  to 
the  manufacture  of  artificial  manures.  The  acid  is  used 
in  making  ammonium  sulphate,  (NH4)2  SO4,  a  salt 
which  is  now  manufactured  in  large  quantities  and  is 
almost  wholly  employed  in  agriculture.  Sulphuric  acid 
is  employed  in  obtaining  hydrofluoric  acid  (of  which 
we  know  something),  nitric  acid,  and  also  stearic  acid, 
which  is  used  in  candle-making.  Sulphuric  acid  is 
indispensable  in  the  manufacture  of  phosphorus,  nitro- 
glycerin,  and  gun-cotton,  substances  to  be  considered 
by  us  in  later  parts  of  the  book.  The  manufacture  of 
aniline  uses  large  quantities  of  this  acid.  Parchment 
paper  is  made  by  immersing  paper  in  sulphuric  acid 
for  a  short  time,  and  then  removing  the  acid  by  con- 
tinued washing  in  water.  The  other  applications  of 
sulphuric  acid  (some  of  them  of  minor  importance) 
are  innumerable. 

Sulphuric   acid   is  clearly  one  of  those   substances 


158     INTRODUCTION   TO   MODERN    CHEMISTRY. 

which  are  manufactured  in  enormous  quantities.  About 
2,156,000  tonnes*  were  made  in  Europe  in  1890  :  this 
represents  about  215,000  railway  waggon  loads;  or 
about  600  loads  (equal  to,  say,  20  goods  trains)  daily  : 
and  the  manufacture  has  increased  greatly  since  1890. 

Many  of  the  salts  of  sulphuric  acid  are  important. 
The  salts  which  it  forms  with  the  heavy  metals  have 
long  been  called  vitriols  ;  iron  vitriol  [also  called 
"  green  vitriol "]  is  iron  sulphate,  copper  vitriol 
[also  known  as  "  blue  vitriol "]  is  copper  sulphate. 
As  either  iron,  Fe,  or  copper,  Cu,  takes  the  place  of 
two  atoms  of  hydrogen  in  acids,  the  formula  of  iron 
sulphate  is  FeSO4,  and  that  of  copper  sulphate  is 
CuSO4. 


ACID  SALTS.     DOUBLE  SALTS.     BASIC  SALTS. 

We  have  now  sufficient  knowledge  to  enable  us  to 
understand  what  yet  remains  to  be  said  about  salts, 
and  especially  about  acid  salts,  double  salts,  and  basic 
salts. 

Acid  salts  are  those  in  which  only  part  of  the 
hydrogen  of  acids  is  replaced  by  metals.  To  take  an 
example.  We  know  that  the  formula  of  the  neutral 
[or  normal]  sodium  sulphate  is  Na2SO4;  this  salt  is 
formed  by  replacing  both  atoms  of  hydrogen  in 
sulphuric  acid,  H2SO4,  by  sodium  ;  but,  as  one  atom 
of  the  metal  sodium  always  replaces  one  atom  of 

*  As  a  metric  tonne  =  -98  ton  (approximately),  the  number  in 
the  text  represents  about  2,113,000  tons.  (TR-I 


ACID   SALTS   AND  DOUBLE   SALTS.  159 

hydrogen,  it  is  easy  to  see  that  another  salt  might 
exist  having  the  formula  NaHSO4.  That  salt  is  an 
acid  salt,  because  it  still  contains  one  atom  of  the 
hydrogen  of  the  acid. 

The  atom  of  hydrogen  in  the  acid  salt  NaHSO4 
can  be  replaced  by  another  metal — by  potassium,  for 
example.  If  this  is  done  we  obtain  the  salt  KNaSO4, 
which  is  a  double  salt,  and  is  called  potassium  sodium 
sulphate.  To  prepare  this  salt,  caustic  potash  is  added 
to  a  solution  of  acid  sodium  sulphate  ;  the  base  (potash) 
neutralises  the  acid  salt  completely,  and  we  obtain  a 
neutral  double  salt.  The  reaction  is  expressed  by  the 
equation  : — 

NaHS04      +         KOH  KNaSO4          +    H2O. 

Acid  sodium     +     caustic  potash     =    potassium  sodium     +     water, 
sulphate  sulphate 

But  there  is  another  kind  of  double  salts — namely, 
such  as  are  produced  when  two  salts  crystallise  from 
a  solution  wherein  they  are  present,  not  one  after 
another,  as  happened  with  benzoic  acid  and  common 
salt  (see  p.  54),  but  combined  together  as  a  double  salt. 
The  number  of  salts  which  crystallise  from  solutions  as 
double  salts  is,  however,  very  small  compared  with  the 
number  of  those  which  always  crystallise  by  themselves 
alone. 

The  alums  are  the  best  known  of  all  double  salts.  The  name 
of  the  class  is  taken  from  common  alum,  which  is  a  double  salt 
of  aluminium  sulphate  and  potassium  sulphate.  One  atom  of 
aluminium  always  takes  the  place  of  three  atoms  of  hydrogen. 
Now,  as  there  are  only  two  atoms  of  hydrogen  in  sulphuric  acid, 


160     INTRODUCTION    TO   MODERN    CHEMISTRY. 

the  adjustment  can  be  brought  about  only  by  the  replacement  of 
six  atoms  of  hydrogen,  from  three  molecules  of  sulphuric  acid, 
by  two  atoms  of  aluminium.  Hence  the  formula  of  aluminium 
sulphate  is  A1,(SO4)3.  The  formula  is  written  A12(SO4)3  to 
indicate  that  three  SO4  [atomic  groups]  are  combined  in  this 
salt  with  two  Al  [two  atoms  of  aluminium].  This  aluminium 
salt,  which  crystallises  very  imperfectly,  combines  eagerly  with 
potassium  sulphate,  whose  formula  is  K2SO4,  to  form  an  easily 
crystallisable  double  salt,  which  has  long  been  known  by  the 
name  alum.  If  the  two  salts  are  present  in  a  solution,  they  do 
not  crystallise  separately,  one  after  the  other,  but  they  crystallise 
out  in  combination,  as  the  double  salt  A12(SO4)3K2SO4.  We  may 
write  this  formula  as  A12K2(SO4)4 ;  we  may  then  halve  this,  and 
so  obtain  the  formula  A1K(SO4)2  for  alum.  But  it  is  much  more 
difficult  to  understand  this  shortened  formula,  because  it  does 
not  make  apparent  the  way  wherein  the  aluminium  and  potas- 
sium, together,  replace  the  four  hydrogen  atoms  of  two  molecules 
of  sulphuric  acid. 

Quite  a  number  of  other  metals  (and  also  ammonium,  con- 
cerning which  we  shall  learn  something  shortly)  can  take  the 
places  of  aluminium  and  potassium  in  the  common  alums. 
Iron  alum,  for  instance,  is  Fe2K2(SO4)4.  The  potassium  in 
this  double  salt  may  be  replaced  by  ammonium,  acting  as  a 
metal  ;  we  thus  obtain  iron  ammonia  alum,  Fe2(NH4)2(SO4)4. 
Almost  all  those  double  salts  which  are  called  alums  crystallise 
very  readily,  so  that  it  is  an  easy  matter  to  obtain  them  pure 
(compare  p.  56).  All  alums  crystallise  with  24  molecules  of 
"  water  of  crystallisation."  If  one  examines  a  crystal  of  common 
alum,  one  finds  that,  besides  A12K2(SO4)4,  it  contains  water,  and 
indeed  24  molecules  of  water.  If  this  water  is  driven  out  by 
heating,  what  is  called  burnt  alum  is  obtained.  Burnt  alum  is 
a  white  amorphous  powder ;  the  crystals  of  alum  are  only 
obtained  when  A12K2(SO4)4  is  combined  with  24H2O.  As  in 
alum,  so  in  very  many  other  crystalline  salts,  there  is  found 
water  of  crystallisation.  In  all  such  cases  the  crystalline  form 
is  impossible  without  the  water.  The  quantity  of  water  of 
crystallisation  can  be  calculated  in  the  same  way  as  the  quantity 
of  any  other  constituent  of  a  chemical  compound.  Let  us 
calculate  the  water  in  alum.  The  atomic  weight  of  aluminium 
jls  27,  of  potassium  39,  of  sulphur  32,  of  oxygen  16,  and  of 


SELENION    AND   TELLURIUM.  l6l 

hydrogen    I.      The    molecular    weight   of    alum    given    by   the 
formula  AlaK2(SO,)424H2O  is— 

A12  K2  (S04X  24H2O 

(2  x  27)  +  (2  x  39)  +  (4  x  96)  +  (24  x  1 8)  =  948. 

In  948  parts  by  weight  of  alum  crystals  there  are  24  x  18  = 
432  parts  by  weight  of  water.     The  proportion — 

432  x  ioo 


948  :  432  =  ioo  :  .r  •   or,  x 


948 


shows  that  alum  contains  45*5  per  cent,  of  water  of  crystallisation. 
Chrome  alum,  Cr2K2(SO4)4  24.H2O,  wherein  chromium  takes  the 
place  of  the  aluminium  of  common  alum,  crystallises  in  especially 
large  and  well-formed  crystals. 

Acid  salts  are  those  salts  which  are  able  to  combine 
with  more  alkali,  after  the  manner  of  acids ;  basic  salts, 
on  the  other  hand,  are  those  salts  wherein  the  whole  of 
the  base  is  not  saturated  by  acid.  Basic  salts  are, 
therefore,  prepared  to  combine  with  more  acid.  The 
formula  of  one  of  these  salts  must  indicate  exactly  the 
quantity  of  base  therein  which  is  not  saturated  by  acid  ; 
and  as  the  relations  of  the  parts  of  such  salts  to  one 
another  vary  much,  the  chemical  compositions,  and 
hence,  of  course,  the  formulae,  of  basic  salts  are  often 
very  complicated. 

The  two  elements  selenion  and  tellurium  are  chemi- 
cally like  sulphur.  Both  occur  only  in  small  quantities. 
Their  likeness  to  sulphur  is  very  marked.  They  burn 
in  air  to  the  compounds  SeO2  and  TeO2  (selenious 
oxide  and  tellurous  oxide),  which  correspond  to  SO2 ; 
in  other  words,  each  of  these  elements,  like  sulphur, 
combines  with  two  atoms  of  oxygen  from  the  air.  The 
two  oxides  SeO2  and  TeO2  can  be  oxidised  to  selenic 

ii 


162      INTRODUCTION    TO   MODERN   CHEMISTRY. 

and  telluric  acids  [H2SeO4  and  HsTeOJ.  If  hydro- 
chloric acid  reacts  with  iron  selenide  or  iron  telluride, 
selenuretted  hydrogen  gas  or  telluretted  hydrogen  gas  is 
obtained,  just  as  sulphuretted  hydrogen  gas  is  produced 
by  the  reaction  of  hydrochloric  acid  with  iron  sulphide. 
These  two  gases  resemble  sulphuretted  hydrogen ;  but 
they  smell  much  more  abominably  and  they  are  much 
more  poisonous  than  that  gas. 

(i)          FeS       +         2HC1       -      FeCl,      +        H,S. 

Iron  sulphide  +  hydrochloric  acid  =  iron  chloride  +   sulphuretted 

hydrogen  gas. 

(ii)        FeSe      +        2HC1        =      FeCl2      +       H2Se. 

Iron  selenide  +  hydrochloric  acid  =  iron  chloride  +    selenuretted 

hydrogen  gas. 

(iii)        FeTe       +         2HC1        =      FeCl2      -f       H2Te. 

Iron  telluride  +  hydrochloric  acid  =  iron  chloride  +     telluretted 

hydrogen  gas. 


WE  pass  now  to  another  group  of  elements,  the 
nitrogen  group.  This  group  contains  four  elements — 
nitrogen,  phosphorus,  arsenic,  and  antimony. 


NITROGEN. 

We  know  (see  p.  120)  that  more  than  77  per  cent,  of 
the  atmosphere  is  composed  of  nitrogen.  The  symbol 
for  nitrogen  is  N ;  the  name  nitrogen  [nitre-producer] 
is  connected  with  nitrum,  the  Latin  name  for  saltpetre. 
We  obtained  nitrogen  by  burning  phosphorus  in  a 
bell-glass  standing  over  water:  the  phosphorus  com- 
bined with  the  oxygen  in  the  air,  forming  phosphoric 
anhydride  (P2O5),  and  the  nitrogen,  which  had  formed 
part  of  the  air  under  the  bell-glass,  remained.  The 
nitrogen  thus  prepared  was  mixed  with  carbon  dioxide, 
argon,  etc.  (see  analysis  of  air  on  p.  120),  for  we  made 
no  attempt  to  remove  these  substances.  But  we  have 
always  sought  to  obtain  the  substances  we  have  ex- 
amined in  a  state  of  purity,  and  we  must  do  the  same 
in  the  case  of  nitrogen,  because  even  small  quantities 
of  impurities  may  greatly  modify  the  properties  of  a 

substance. 

163 


164      INTRODUCTION    TO   MODERN    CHEMISTRY. 

To  obtain  pure  nitrogen  we  employ  a  method  very 
different  from  that  we  used  for  making  impure  nitrogen 
from  the  air. 

We  know  that  the  formula  of  nitric  acid  is  HNOs. 
There  exists  an  acid,  called  nitrous  acid,  which  is  related 
to  nitric  acid  in  the  same  way  as  sulphurous  acid 


Fig.  42.— Preparation  of  nitrogen  gas. 

(H2SO3)  is  related  to  sulphuric  acid  (H2SO4)  :  as 
sulphurous  acid  contains  one  atom  of  oxygen  less  than 
sulphuric  acid  so  nitrous  acid  is  poorer  in  oxygen  than 
nitric  acid  by  one  atom.  The  formula  of  nitrous  acid 
must,  then,  be  HNO2.  This  acid  reacts  with  bases  and 
forms  salts ;  with  ammonia  it  forms  ammonium  nitrite, 
NH4NO2.  We  must  know  something  of  ammonia  (we 
come  to  that  compound  in  a  few  pages)  before  we  are 


PREPARATION   OF   NITROGEN.  165 

in  a  position  to  understand  why  the  salts  of  ammonia 
are  spoken  of  as  ammonium  salts.  Looking  more 
clearly  at  the  formula  of  ammonium  nitrite,  NH4NO;z, 
we  notice  that  the  salt  is  composed  of  two  atoms  of 
nitrogen,  united  to  four  atoms  of  hydrogen  and  two 
atoms  of  oxygen — that  is,  to  twice  two  hydrogen  atoms 
and  one  oxygen  atom.  But  the  formula  of  water,  H2O, 
is  two  hydrogen  and  one  oxygen  atoms.  Hence  we 
can  very  well  understand  that  ammonium  nitrite  might 
decompose  to  two  molecules  of  water  (2H2O)  and  one 
molecule  of  nitrogen  (N2).  This  decomposition  is, 
indeed,  easily  effected  by  heat.  It  is  only  necessary  to 
boil  a  solution  of  the  salt  in  water ;  the  nitrogen  comes 
off  plentifully  from  the  boiling  liquid,  and  may  be 
collected  in  the  way  shown  in  fig.  42,  where  A  repre- 
sents the  flask  wherein  the  solution  of  ammonium 
nitrite  is  heated.  The  equation  that  expresses  the 
reaction  is  the  following  : — 

NH4NO2        =        N2          -f2H2O. 
Ammonium  nitrite  =  nitrogen  gas  -f  water. 

This  reaction  yields  pure  nitrogen. 

The  simplest  way  of  stating  the  behaviour  of  nitrogen 
is  to  say  that  it  is  just  the  opposite  of  that  of  oxygen. 
For  example,  a  glimmering  chip  of  wood  takes  fire  in 
oxygen,  but  a  brightly  burning  chip  is  extinguished 
when  it  is  immersed  in  a  cylinder  filled  with  nitrogen. 

In  speaking  of  the  preparation  of  nitrogen  we  learned 
that  nitric  acid  is  richer  in  oxygen  by  one  atom  than 
nitrous  acid  ;  hence  ammonium  nitrate  is  richer  in 


166      INTRODUCTION    TO   MODERN    CHEMISTRY. 

oxygen  by  one  atom  than  ammonium  nitrite,  and 
must  therefore  have  the  formula  NH4NO3.  Now  what 
will  happen  when  this  salt  is  heated  ?  How  will  the 
atom  of  oxygen  behave  that  is  in  this  salt  over  and 
above  the  number  of  atoms  of  oxygen  in  ammonium 
nitrite  ?  Experiment  shows  (as  was  to  be  expected) 
that  the  water  is  not  further  oxidised  by  the  extra 
oxygen  atom,  but  that  this  oxygen  holds  to  the 
nitrogen,  and,  combined  therewith,  forms  N2O.  The 
compound  that  is  obtained  thus  without  difficulty  is 
called  nitrous  oxide  gas. 

NH4N03       =  N20  +  2H20. 

Ammonium  nitrate  =  nitrous  oxide  gas  +  water. 

Nitrous  oxide  is  sometimes  called  "  laughing  gas."  When 
breathed,  it  produces  a  condition  of  semi-unconsciousness,  which 
lasts  for  a  short  time — long  enough,  for  instance,  to  allow  of  the 
extraction  of  a  tooth.  This  gas  has  been  used,  and  is  still  to 
some  extent  used,  in  dentistry,  although  more  convenient  sub- 
stances are  now  at  our  disposal — ethyl  chloride,  for  example — 
which  cause  insensibility  only  around  the  tooth  to  be  operated  on, 
and  have  the  great  advantage  of  not  requiring  that  complete  un- 
consciousness should  be  produced  for  the  performance  of  a  slight 
operation.  And  so  it  is  that  the  interest  in  nitrous  oxide  gas  of 
those  who  are  not  chemists  is  gradually  passing  away. 

In  the  preparation  of  nitrogen  and  nitrous  oxide  we 
had  two  exceedingly  clear  examples  of  how  it  is  some- 
times possible  to  tell,  from  the  inspection  of  the  formula 
of  a  compound,  what  are  likely  to  be  the  products 
of  the  decomposition  of  that  compound.  In  the  cases 
in  question  the  compounds  were  ammonium  nitrite 
(NH4NO2)  and  ammonium  nitrate  (NH4NO3\ 

We  shall  now  return  to  our  usual  custom  of  becoming 


PREPARATION    OF   AMMONIA.  167 

acquainted  with  the  most  important  hydrogen  com- 
pound of  the  element  we  are  considering.  Ammonia 
is  the  chief  compound  which  nitrogen  forms  with 
hydrogen.  The  name  has  come  down  from  old  times. 
Ammonia  is  a  compound  of  one  atom  of  nitrogen  and 
three  atoms  of  hydrogen;  its  formula  is  NH3.  This 
compound  is  formed  whenever  dry  nitrogenous  materials 
are  heated  or  are  allowed  to  putrefy.  It  was  by  one 
of  these  methods — namely,  by  heating  a  mixture  of 
camel's  dung  and  salt  (sodium  chloride)  in  earthen  jars 
— that  ammonium  chloride,  or  salammoniac  as  it  has 
been  called  since  ancient  times,  was  prepared  long  ago 
in  the  East. 

Ammonia — the  compound  NH3 — is  a  gas  with  basic 
properties,  and  hence  it  turns  red  litmus  paper  blue. 
This  gas  is  quite  the  reverse  of  such  a  gas  as  hydro- 
chloric acid,  which  has  acid  properties.  These  two 
gases  combine  to  form  a  salt — namely,  ammonium 
chloride  or  salammoniac.  We  are  not  at  all  surprised 
to  find  that  the  solid  body  salammoniac  should  be 
composed  of  two  gases ;  we  know  that  water,  and 
hence,  of  course,  ice,  consists  of  two  gases. 

Like  most  other  gases,  ammonia  is  colourless.  It  has 
a  very  penetrating  smell.  The  compound  is  obtained 
by  heating,  in  a  flask,  a  mixture  of  salammoniac  and 
a  base  stronger  than  ammonia.  As  we  regarded 
common  salt  as  sodium  hydrochloride  (p.  67),  so  may 
we  look  on  salammoniac  as  ammonium  hydrochloride. 
The  stronger  base  will  drive  out  the  ammonia  gas,  and 

*  This  is  a  contraction  either  of  sal  ammoniacum  or  of  sal 
armeniacum.  There  are  doubts  as  to  which  is  correct. 


1 68      INTRODUCTION   TO   MODERN   CHEMISTRY. 

will  combine  with  the  hydrochloric  acid  in  place  of  the 
ammonia.  To  do  this  we  shall  use  that  base  which  is 
at  once  the  cheapest  and  the  most  effective — namely, 
burnt  lime  that  has  been  slaked  by  a  little  water.  In 
the  reaction  of  this  base  with  salammoniac  the  base 
combines  with  the  hydrochloric  acid,  taking  the  place 
of  the  ammonia  and  producing  calcium  chloride,  while 
the  ammonia  which  is  set  free  escapes  as  a  gas 
from  the  flask.  The  following  equation  represents  the 
reaction  : — 

2NH4C1      +    Ca(OH)2    =       2NH3         +    CaCl,  +  2H./X 

Salammoniac  +    slaked  lime  =    ammonia  gas  +  calcium  +  water. 

chloride 

Ammonia  gas  is  exceedingly  soluble  in  water,  and 
therein  it  resembles  hydrochloric  acid  gas  ;  hence,  like 
hydrochloric  acid  gas,  it  cannot  be  collected  over  water, 
as  it  would  dissolve  in  the  water  instead  of  collecting  in 
the  cylinder.  If  we  wish  to  examine  ammonia  gas, 
we  must  collect  it  over  mercury,  using  that  liquid  in 
place  of  water.  Fig.  43  shows  an  apparatus  suited  for 
this  purpose.  The  cylinder  is  filled  with  mercury,  and 
inverted  in  mercury  in  a  pneumatic  trough  (made  as 
small  as  possible  to  save  mercury,  which  is  expensive), 
and  the  ammonia  is  led  into  this  cylinder  from  the 
mixture  in  the  flask  A,  wherefrom  it  is  produced. 

If  ammonia  gas  is  led  into  water,  which  may  be 
placed  in  a  washing-flask  (in  an  arrangement  similar 
to  that  shown  on  p.  68),  the  water  absorbs  a  large 
quantity  of  the  ammonia,  and  we  obtain  ammonia 
liquor — that  is,  an  aqueous  solution  of  the  gas.  A  litre 
of  such  a  solution  contains  more  than  300  grams 


COMPOUNDS   OF   AMMONIA.  169 

of  ammonia  [nearly  2O,OOO  grains  per  gallon].  An 
aqueous  solution  of  ammonia  is  popularly  known  as 
"  spirit  of  hartshorn." 

Practically  all  the  ammonia  that  is  used  nowadays  is 
obtained  from  the  products  of  the  gas-works,  so  that 
it  is  not  necessary  to  have  recourse  to  salammoniac 
from  the  East.  We  shall  discover  the  reason  for  this 


Fig.  43.— Collection  of  ammonia  gas  over  mercury. 

when  we  consider  the  manufacture  of  coal-gas ;  and  we 
shall  also  see,  in  a  later  part  of  the  book,  why  almost 
all  the  ammonia  that  can  be  produced  by  the  gas-works 
in  all  parts  of  the  world  is  used  as  artificial  manure. 

Ammonia  combines  with  sulphuric  acid  to  form 
sulphate  of  ammonia,  or,  as  it  is  now  called,  ammonium 
sulphate.  The  formula  of  this  salt  is  (NH4)2SO4;  corre- 
sponding closely  to  that  of  potassium  sulphate,  K2SO4. 


I/O      INTRODUCTION   TO   MODERN   CHEMISTRY. 

Ammonia  gas  has  the  formula  NH3 ;  it  may  be  re- 
garded as  the  anhydride  of  a  base,  just  as  SO2  is  regarded 
as  the  anhydride  of  the  corresponding  sulphurous  acid, 
H2SO3.  The  behaviour  of  an  aqueous  solution  of 
ammonia  towards  acids  is  as  if  the  ammonia  had 
combined  with  the  water  and  the  solution  contained 
the  compound  NH4OH.  This  corresponds  with  the 
behaviour  of  an  aqueous  solution  of  SO2  towards  bases. 
When  an  aqueous  solution  of  ammonia  is  added  to 
diluted  sulphuric  acid,*  we  may  regard  the  reaction 
as  taking  place  thus : — 

2NH4OH  +        H2SO4    *  =         (NH4)2SO4          +  2H2O ; 
Ammonia  +  sulphuric  acid  =  ammonium  sulphate  +  water ; 

just  as  the  reaction  between  solutions  of  caustic  potash 
and  sulphuric  acid  is  this  : — 

2KOH         +       H2SO4        =  K2SO4  +  2H2O. 

Caustic  potash  +  sulphuric  acid  =  potassium  sulphate  -}-  water. 

If  the  formula  of  ammonia  is  written  in  the  foregoing 
equations  as  that  of  the  gas,  NH3,  then  the  equations 
represent  the  changes  as  being  direct  additions  of  the 
ammonia  to  the  acids  ;  thus  :-- 

NH3      +  HC1  =      NH4C1. 

Ammonia  +  hydrochloric  acid  =  salammoniac. 

2NH3     +        H2S04        =         (NH4)2S04. 
Ammonia  +  sulphuric  acid  =  ammonium  sulphate. 

But  this  is  against  the  generalisation  (see  p.  150) 
that  the  reaction  between  an  acid  and  a  base  always 

*  The  sulphuric  acid  is  diluted  to  moderate  the  reaction 
between  the  acid  and  the  base. 


AMMONIUM   COMPOUNDS.  i;i 

produces  water  as  well  as  a  salt.  Such  an  insufficient 
way  of  writing  the  reactions  of  ammonia  with  acids 
as  is  used  in  the  two  preceding  equations  is  often  a 
stumbling-block  which  beginners  in  chemistry  do  not 
know  how  to  remove.  It  is  therefore  much  better 
for  beginners  always  to  write  ammonia  in  such  equa- 
tions, not  as  the  anhydride  NH3,  but  as  NH4OH,  for 
by  doing  so  all  difficulties  disappear. 

The  atomic  complex  NH4,  which,  however,  cannot 
itself  be  isolated  (see  later,  under  valency),  is  called 
ammonium,  reminding  one  of  potassium  and  sodium, 
which  it  closely  resembles ;  and  the  salts  that  are 
formed  from  ammonia  and  acids  are  called  ammonium 
salts  (ammonium  sulphate,  ammonium  acetate,  etc.), 

The  compound  of  ammonia  gas  with  carbonic  anhy- 
dride gas  (CO2)  is  sometimes  used  for  making  cakes 
light  and  spongy,  instead  of  the  more  tedious  process  of 
fermentation  by  yeast.  The  action  of  the  salt  in  causing 
dough  to  rise  is  as  follows.  Although  the  two  gases 
ammonia  and  carbonic  anhydride  unite  at  the  ordinary 
temperature  to  form  a  solid  white  salt,  yet  at  the 
temperature  of  a  baking  oven  the  salt  is  decomposed 
into  its  two  constituents  ;  and  it  is  these  two  gases 
(ammonia  and  carbon  dioxide)  which  bubble  through 
the  dough  and  so  make  it  rise.* 

*  The  salt  generally  employed  as  a  baking  powder  is  not  at 
all  the  normal  ammonium  carbonate.  The  acid  anhydride  CO2  and 
the  basic  anhydride  NH3  cannot  form  the  normal  salt  [which 
would  be  (NH4),CO3],  as  water  is  not  present.  Their  reaction 
may  be  expressed  thus  : — 

CO2  +     2NH3    =          (NH4)CO2NH2. 

Carbonic  anhydride  +  ammonia  =  ammonium  amido  carbonate. 


172      INTRODUCTION    TO   MODERN    CHEMISTRY. 

Two  other  compounds  of  nitrogen  and  hydrogen  have  been 
prepared  recently:  one  has  the  formula  N3H,  and  is  called 
hydrazoic  acid;  the  other  has  the  formula  N2H4,  and  is  named 
hydrazine. 

Nitrogen  combines  with  oxygen  in  five  proportions. 
We  have  already  become  acquainted  with  one  of  the 
compounds,  N2O,  nitrous  oxide ;  and  we  have  also 
alluded  to  nitrous  and  nitric  acids.  We  must  now 
study  nitric  acid  pretty  fully ;  but  we  shall  leave  on 
one  side,  as  not  particularly  important  for  our  purposes, 
the  two  oxides  NO  (nitric  oxide)  and  NO2  (nitrogen 
dioxide). 

NITRIC  ACID. 

We  should  not  expect  to  find  nitric  acid  uncombined 
on  the  earth's  surface ;  so  strong  an  acid  will  always 
meet  with  bases  wherewith  it  can  combine.  Two  salts 
of  nitric  acid  are  found  very  abundantly — potassium 
nitrate  in  India  and  sodium  nitrate  in  Chili.  The 
former  is  called  potash  saltpetre  [or  nitre\,  and  the 
latter  soda  saltpetre  [or  Chili  saltpetre'].  The  name  salt- 
petre is  used  to  include  both.  These  two  salts,  both 
of  which  are  soluble  in  water,  are  found  in  the  soil  of 
certain  districts  in  India  and  Chili ;  if  these  soils  are 
boiled  with  water,  the  salts  dissolve,  and  are  obtained 
as  crystals  by  pouring  off  the  solutions  from  the  soils, 
evaporating,  and  allowing  to  cool. 

Potash  saltpetre  has  been  known  in  Europe  since 
about  the  fifth  century.  Its  composition  is  expressed 
by  the  formula  KNO3,  from  which  we  calculate  the 
percentage  of  oxygen  in  the  salt  to  be  47 -5.  For, 


PREPARATION   OF   NITRIC   ACID.  173 

the  atomic  weights  of  potassium,  nitrogen,  and  oxygen 
being  39,  14,  and  16  respectively,  we  have  : — 

K        N  '         03 

39  +  14  +  (3  x  16)  =  101  ; 

hence  101  :  48  =  100  :  x  ;     x  =  - — ^ =  47-5. 

The  salt  is  evidently  rich  in  oxygen. 

To  prepare  nitric  acid  from  nitre  the  salt  is  heated 
with  sulphuric  acid  ;  the  sulphuric  acid  drives  out  the 
nitric  acid  and  combines  with  the  potassium  in  its 
place.  The  reaction  is  expressed  in  the  following 
equation  : — 

KN03          +         H2S04       =      HN03     +         KHSO4. 
Potassium  nitrate  +  sulphuric  acid  =  nitric  acid  +  acid  potassium 

sulphate. 

As  nitric  acid  is  a  liquid  which  can  be  distilled,  we 
use  a  retort  in  preparing  it,  and  this  makes  it  easy  to 
separate  the  acid  from  the  acid  potassium  sulphate. 
The  potassium  nitrate  and  the  sulphuric  acid  are  placed 
in  a  retort  (R,  fig.  44),  which  is  then  heated ;  vapours 
soon  come  off,  and  condense  to  a  liquid  in  the  receiver 
A,  which  is  placed  in  a  basin  filled  with  cold  water,  and 
is  thereby  cooled  sufficiently  for  our  purpose.  The 
liquid  that  condenses  in  the  receiver  is  nitric  acid  ;  the 
acid  potassium  sulphate  remains  in  the  retort. 

In  preparing  nitric  acid  we  meet  with  an  acid  salt — namely, 
acid  potassium  sulphate.  This  is  a  salt  formed  by  replacing 
only  a  portion  of  that  hydrogen  of  the  acid  which  is  replaceable 
by  metal;  hence  the  salt  KHSO4  is  still  acid,  and  is  able  to 
react  with  bases  after  the  manner  of  an  acid.  This  salt  is  also 


174      INTRODUCTION   TO   MODERN    CHEMISTRY. 

able  to  decompose  potassium  nitrate  after  the  manner  of  free 
sulphuric  acid;  producing,  finally,  neutral  potassium  sulphate, 
and  setting  free  the  nitric  acid  which  was  combined  with  the 
second  potassium  atom  that  seized  the  acid  potassium  sulphate 
in  order  to  convert  it  into  the  neutral  salt. 


HKS04       +KN03=  K2SO,  +      HNO3. 

Acid  potassium  +    nitre    =  neutral  potassium  +  nitric  acid 
sulphate  sulphate  (red  fuming). 

As  one  would  expect,  acid  potassium  sulphate,  with  its  half- 
neutralised  sulphuric  acid,  does  not  react  with  nitre  so  readily 


Fig.  44.— Preparation  of  nitric  acid. 

as  free  sulphuric  acid  reacts.  Hence,  if  acid  potassium  sulphate 
is  to  expel  nitric  acid  from  nitre  (in  accordance  with  the  equation 
given  above),  the  temperature  of  the  mixture  of  nitre  and  acid 
potassium  sulphate  must  be  made  much  higher  than  is  required 
when  sulphuric  acid  itself  is  employed.  This  is  a  matter  of 
no  moment  in  itself.  But  nitric  acid  is  no  longer  completely 
stable  at  this  higher  temperature  ;  it  cannot  suffer  this  high 
temperature  without  taking  hurt.  A  portion  of  the  acid,  it  is 
true,  distils  over  as  such,  as  HNO3 ;  but  another  portion  decom- 
poses to  simpler  compounds,  and  particularly  to  nitrogen  dioxide, 
NO0.  The  nitrogen  dioxide,  which  is  a  red  gas,  dissolves  in 


RED   FUMING   NITRIC   ACID.  175 

that  part  of  the  nitric  acid  that  distils  over  unchanged.  Hence 
the  nitric  acid  in  the  receiver  appears  red ;  and  the  liquid  fumes 
if  the  flask  is  opened,  because  the  red  gas  evaporates  into  the  air 
from  the  liquid.  The  liquid  in  the  receiver  is  called  red  fuming 
nitric  acid. 

The  final  result  of  these  considerations  is  as  follows.  If  it  is 
desired,  as  it  generally  is,  to  make  colourless  nitric  acid,  the 
quantities  of  nitre  and  sulphuric  acid  must  be  such  that  the 
whole  of  the  nitre  is  decomposed  by  the  sulphuric  acid,  and, 
consequently,  acid  potassium  sulphate  remains  in  the  retort. 
The  equation,  which  has  been  given  already — 

KNO3  +       H2SO4        -  HKSO4  +     HNO3 

nitre     +  sulphuric  acid  =-  acid  potassium  sulphate  -f  nitric  acid 

tells  tl^t- 
IC        N  03 

39  +  14  4  (3  x   16)  =  101  parts  by  weight  of  nitre,  and 
H,      S  O4 

2  +  32  +  (4  x    16)  =  98  parts  by  weight  of  sulphuric  acid 

(or  97  parts  by  weight  of  sulphuric  acid  for  100  parts  of  nitre) 
must  be  used.  But  if  it  is  desired  to  prepare  the  red  fuming 
nitric  acid,  which  is  often  used  as  an  oxidising  agent,  the  forma- 
tion of  neutral  potassium  sulphate  must  be  kept  in  view,  and  half 
as  much  sulphuric  acid  must  be  employed  for  the  same  quantity 
of  nitre  as  before  [or  twice  as  much  nitre  for  the  same  quantity 
of  sulphuric  acid],  in  accordance  with  the  equation  : — 

2KNO3  +       H2SO4       =          K2SO4  +  2HNO3. 

Nitre  -r  sulphuric  acid  =  neutral  potassium  +  red  fuming  nitric  acid, 
sulphate 

In  this  case  it  is  necessary  to  use  2  x  101  =  202  parts  by  weight 
of  nitre  to  98  parts  of  sulphuric  acid,  or  only  48*5  parts  by  weight 
of  sulphuric  acid  for  100  parts  of  nitre ;  for  when  the  sulphuric 
acid  has  driven  out  the  first  half  of  the  nitric  acid,  and  has  thus 
been  changed  to  acid  potassium  sulphate,  then  this  acid  salt  must 
react  with  the  second  half  of  the  nitre,  with  the  total  result  that 
red  fuming  nitric  acid  is  obtained. 


1 7(5      INTRODUCTION   TO   MODERN    CHEMISTRY. 

Nitric  acid  is  a  very  strong  acid.  It  dissolves  all 
metals  except  gold  and  platinum.  As  it  dissolves 
silver,  nitric  acid  may  be  used  to  separate  silver  from 
gold  ;  hence  its  older  name,  parting  acid.  The  readi- 
ness wherewith  silver  dissolves  in  nitric  acid  is  shown 
by  the  following  experiment.  As  it  has  been  found  in 
practice  that  concentrated  nitric  acid  diluted  with  20  per 
cent,  of  water  is  the  most  suitable  for  the  purpose,  acid 
of  that  concentration  is  put  in  the  flask  A  (fig.  45),  the 
flask  is  very  slightly  warmed,  and  a  little  piece  of  silver 
foil  is  thrown  into  it.  The  silver  dis- 
solves in  the  liquid,  and  disappears 
almost  instantly.  The  acid  and  the 
metal  interact  to  form  silver  nitrate; 
hence  it  is  only  necessary  to  evaporate 
the  contents  of  the  flask  to  dryness — in 
other  words,  to  remove  the  acid  over 

Fig.  45-  — Dis- 
solving silver  and    above   what    was    used    in    the    re- 
in nitric  add.    action_in    order    to    obtain    the    silver 

nitrate  as  a  solid  residue. 

Silver  nitrate  is  white,  like  so  many  salts ;  its 
formula  is  AgNO3.  This  salt,  an  aqueous  solution 
whereof  we  have  already  often  used  (see  pp.  9, 
50,  146),  was  once  called  lapis  mfernah's,  an  as- 
tonishing name  derived  from  the  fact  that  anything 
touched  with  a  solution  of  the  substance  very  soon 
becomes  black.  The  blackening  effect  is  due  to  the 
decomposition  of  the  salt  into  its  constituents  by 
light,  and  hence  the  separation  of  the  silver  as  an 
exceedingly  fine  black  powder  on  any  surface  which 
has  been  rubbed  with  the  salt  and  exposed  to 
light. 


EXPLOSIVES.  177 

AQUA  REGIA. 

We  have  just  learned  that  gold  and  platinum  are 
insoluble  in  nitric  acid ;  there  is  no  single  acid  which 
dissolves  these  metals,  but  a  mixture  of  nitric  and 
hydrochloric  acids  brings  them  into  solution  :  as  the 
alchemists  called  gold  the  king  of  metals,  they  gave 
the  name  aqua  regia  to  this  mixture.  Nitric  acid 
(HNO3)  is  very  rich  in  oxygen,  and  hydrochloric  acid 
(HC1)  consists  of  hydrogen  and  chlorine ;  when  these 
two  are  mixed,  the  oxygen  of  the  nitric  acid  exerts  an 
oxidising  action  on  the  hydrogen  of  the  hydrochloric 
acid,  combining  with  it  to  form  water,  so  that  the 
chlorine  becomes  available.  We  know  that  chlorine 
is  extraordinarily  active — or,  to  use  a  better  expression, 
that  chlorine  possesses  much  chemical  energy ;  hence 
we  are  not  surprised  that  the  chlorine,  as  it  is  produced 
in  the  nascent  state,  should  attack  gold  arid  platinum. 
If  a  solution  of  gold  or  platinum  in  aqua  regia  is 
evaporated  to  dryness,  gold  chloride  or  platinum 
chloride  is  obtained.  These  salts  dissolve  in  water 
very  easily. 

We  must  just  mention  that  it  has  been  found,  in  recent  years, 
that  a  solution  of  potassium  cyanide  (which  substance  will  be 
considered  when  we  come  to  the  compounds  of  carbon)  dissolves 
gold,  but  not  platinum.  This  method  of  extracting  gold  from  the 
finely  powdered  rock  is  now  used  in  South  Africa,  almost  to  the 
exclusion  of  all  others. 

EXPLOSIVES. 

We  know  that  the  salts  of  nitric  acid  are  very  rich 
in  oxygen.  It  is  its  richness  in  oxygen  that  has  caused 

12 


178      INTRODUCTION    TO   MODERN   CHEMISTRY. 

potassium  nitrate — a  substance  which  does  not  differ  in 
appearance  from  a  thousand  other  white  salts — to  have 
a  greater  influence  on  the  history  of  peoples  than  any 
other  chemical  compound.  For  the  history  of  a  people 
has  often  depended  on  a  battle  won  or  lost,  and  it 
is  this  nitre  which  has  helped  to  decide  innumerable 
battles  of  the  later  middle  ages  and  of  modern  times. 
Nitre  has  been  the  foundation  of  all  sorts  of  gun- 
powder until  very  recently.  For  about  fifteen  years 
attempts  have  been  made  to  replace  powder  made 
with  potassium  nitrate  by  smokeless  explosives  based 
on  nitric  acid. 

The  connection  between  nitre  and  gunpowder  is  as 
follows.  A  mixture  of  nitre  with  such  combustible 
substances  as  charcoal,  pitch,  or  sulphur  continues  to 
burn  once  it  has  been  ignited.  In  distinction  to  ordinary 
combustibles,  such  a  mixture  does  not  require  to  get 
oxygen  from  the  air  for  its  combustion,  but  it  finds  the 
required  oxygen  in  the  nitre.  Hence  such  a  mixture 
can  do  what  an  ordinary  combustible  body  cannot  do : 
it  can  burn  in  an  enclosed  space  without  the  entrance 
of  oxygen  from  outside ;  it  can  burn  under  water,  for 
instance. 

We  have  here  a  mixture  of  25  parts  ordinary  gun- 
powder, 50  parts  nitre,  40  parts  charcoal,  and  10  parts 
sulphur,  which  has  been  moistened  with  spirit  (this 
removes  the  risk  of  explosion  during  the  rubbing), 
well  mixed  by  rubbing,  packed  as  tightly  as  possible 
into  a  paper  covering,  and  kept  for  some  time  in  a 
chamber  at  110°,  whereby  the  spirit  has  been  re- 
moved. This  cartridge  will  be  about  7  cm.  long  by 
4  mm.  diameter  [say,  3  in.  by  J  in.]  ;  if  we  hold  it 


COMBUSTION   OF   GUNPOWDER.  179 

by  a  pair  of  tongs,  ignite  it,  and  bring  it  under  a 
cylinder  filled  with  water  and  inverted  in  water  in  the 
pneumatic  trough,  as  shown  in  fig.  46,  we  shall  see 
that  the  cartridge  continues  to  burn  under  the  water, 
and  that  the  cylinder  is  filled  rapidly  with  the  gases 
that  are  produced  by  burning  our  mixture.  In  this 
experiment  the  cylinder  should  not  stand  on  the  shelf 


Fig.  46.— Burning  gunpowder  under  water,  and  collecting  gases  produced. 

of  the  trough,  but  should  be  supported  by  a  clamp  (as 
shown  in  fig.  46).  Ordinary  gunpowder  is  not  suitable 
for  the  experiment,  because  it  burns  much  too  rapidly, 
and  one  does  not  get  sufficient  time  to  bring  a  cartridge 
filled  with  that  powder  under  the  water  after  it  has 
been  ignited. 

As  long  ago  as  the  seventh  century,  pots  filled  with 
a  mixture  of  nitre  and  combustible  bodies,  which  was 


180      INTRODUCTION    TO   MODERN   CHEMISTRY. 

set  on  fire,  were  thrown  as  dreadful  weapons  against 
the  enemy  or  the  enemy's  ships.  This  was  called 
"  Greek  fire."  It  was  hundreds  of  years  after  this, 
however,  that  men  came  to  recognise  the  fact,  and  to 
turn  it  to  practical  account,  that  there  was  something 
in  this  mixture  besides  its  astonishing  combustibility— 
that  the  mixture  had  an  explosive  power.  By  explosive 
power  we  mean  a  power  which  is  suddenly  developed 
and  is  capable  of  scattering  the  surrounding  objects 
in  all  directions.  Records  dating  from  the  thirteenth 
century  show  that  it  had  been  discovered  by  that  time 
that  the  best  mixture  for  explosive  purposes  was  one 
of  nitre,  charcoal,  and  sulphur,  the  same  mixture  as 
that  we  now  call  gunpowder.  A  military  gunpowder 
made  in  the  eighties  of  the  nineteenth  century  consisted 
of  74  per  cent,  nitre,  16  per  cent,  charcoal,  and  10  per 
cent,  sulphur. 

There  is  nothing  now  mysterious  about  the  fact  that 
such  a  mixture  of  nitre,  charcoal,  and  sulphur  drives  a 
shot  from  a  cannon  or  splits  the  rock  around  the  hole 
wherein  it  is  placed.  The  reason  is  easy  to  understand. 
The  mixture  of  the  three  substances  is  placed  behind 
the  shot  or  in  the  bore-hole  in  the  rock  ;  there  it 
occupies  comparatively  a  very  small  space.  But 
when  the  mixture  is  fired,  there  is  suddenly  formed 
carbonic  acid  gas  (CO2)  from  the  carbon  (C),  and 
sulphur  dioxide  gas  (SO2) — along  with  some  sulphuric 
acid — from  the  sulphur  (S).  The  nitre,  KNO3,  gives 
the  necessary  oxygen.  This  compound  is,  of  course, 
decomposed,  and  its  nitrogen  is  set  free  as  gas.  The 
potassium  of  the  nitre  reacts  with  the  gases,  and  most 


COMBUSTION   OF  GUNPOWDER.  l8l 

of  it  is  changed  to  potassium  carbonate  and  sulphite 
(some  of  the  latter  becoming  sulphate),  which  are  thrown 
into  the  air.  As  these  salts  are  in  a  state  of  ex- 
tremely fine  division,  they  float  in  the  air  for  some 
time,  and  form  the  white  smoke  of  powder.  To 
sum  up :  a  small  quantity  of  powder,  when  burnt, 
produces  a  very  large  quantity  of  gases,  leaving  the 
powder-vapour  out  of  account.  As  these  gases,  at 
the  moment  of  their  formation,  are  tremendously  com- 
pressed in  a  small  space,  they  seek  every  opportunity 
of  expanding.  The  movable  shot  lies  on  one  side  of 
the  gases ;  hence,  to  gain  space,  they  drive  the  shot 
out  of  the  gun.  The  strength  of  the  cannon,  or  the 
gun,  withstands  the  shock ;  but  if  the  gases  are  pro- 
duced in  a  bore-hole,  they  shatter  the  surrounding 
materials. 

It  should  be  noticed  that  experiments  have  shown  that  sodium 
nitrate  cannot  be  substituted  for  potassium  nitrate  in  gunpowder. 
Although  the  two  salts  are  very  like  one  another,  yet  they  are 
not  identical  in  their  behaviour. 


The  preparation  of  gunpowder  from  its  three  con- 
stituents has  been  so  perfected  in  the  last  forty  years 
that  further  improvements  are  scarcely  conceivable,  and, 
indeed,  for  many  reasons,  are  not  necessary.  Cannon 
shots  can  be  projected,  by  the  help  of  gunpowder, 
through  distances  greater  than  twenty  kilometres  [over 
twelve  and  a  half  miles]. 

On  the  other  hand,  the  conditions  regarding  guns 
are  very  different.  It  was  demonstrated,  in  the  fifties, 
by  purely  mathematical  deductions,  that,  if  the  diameter 
of  projectiles  were  diminished  as  much  as  possible,  such 


1 82      INTRODUCTION   TO   MODERN   CHEMISTRY. 

projectiles  could  be  sent  to  much  greater  distances  than 
before,  provided  an  impulse  could  be  given  to  the  con- 
siderably reduced  shots  stronger  than  that  generally 
obtainable  from  the  explosive  power  of  the  old  powder. 
Now,  explosives  have  long  been  known  in  chemical 
laboratories  whose  power  is  much  greater  than  that  of 
the  gunpowder  now  in  use.  For  instance,  a  substance 
called  fulminating  mercury  is  obtained  by  the  action  of 
nitric  acid  on  mercury  in  the  presence  of  alcohol.  (We 
shall  not  concern  ourselves  with  the  composition  and 
the  formula  of  this  very  complex  compound.)  This 
substance  is  exploded  by  any  blow  ;  hence  it  is  too 
dangerous  as  a  substitute  for  gunpowder :  moreover, 
it  explodes  so  suddenly  that  if  it  were  used  in  a  gun 
it  would  burst  the  gun  before  it  set  the  shot  in  motion. 
It  is  not  every  explosive  that  can  be  used  in  place 
of  gunpowder,  or  as  a  bursting  material  in  mining 
operations. 

A  substance  was  discovered  in  the  forties  which 
made  possible,  at  a  later  time,  the  preparation  of  a  new 
powder,  more  powerful,  and  relatively  less  dangerous 
to  manipulate,  'than  the  old  gunpowder.  Cellulose  is 
required  for  the  preparation  of  this  substance.  We 
have  already  mentioned  cellulose  when  speaking  of 
paper  (see  p.  121),  and  we  know  that  the  formula  of 
this  compound  is  C6H10O5.  Cellulose  is  a  constituent 
of  all  plants.  Paper  is  made  from  rags  which  have 
come  from  substances  manufactured  from  plant- fibres 
— from  linen  rags,  for  instance — so  that  the  presence  of 
cellulose  in  paper  is  not  astonishing.  Cotton  wool  is 
nearly  pure  cellulose. 


FORMULA   OF   ACIDS   AND   BASES.  183 

To  understand  what  follows  we  must  once  more 
recall  the  formulae  of  acids  and  bases;  for  example, 
the  formulae  of  sulphuric  acid,  H2SO4,  and  nitric  acid, 
HNO3,  and  the  formulae  of  caustic  potash  and  caustic 
soda,  KOH  and  NaOH.  In  all  of  these  we  find  the 
atomic  group  OH,  composed  of  an  atom  of  oxygen  and 
an  atom  of  hydrogen.  To  recognise  this  group  in  the 
formulae  of  the  two  acids  we  must  group  the  atoms  that 
form  these  acids  in  certain  ways,  and  write — 

SO2    <Q^  in  place  of  H2SO4,  and 
NO*  —  OH  in  place  of  HNO3. 

We  shall  soon  become  acquainted  with  the  hypotheses 
which  enable  us  to  form  a  clear  mental  picture  of  the 
grouping  of  atoms  in  compounds  ;  we  shall  discover 
that  these  hypotheses  are  not  hard  to  grasp — not  so 
hard,  indeed,  as  the  doctrine  of  atoms  and  their  weights. 
The  group  OH  is  very  ready  to  combine  with  another 
atom  of  hydrogen,  so  forming  water,  H2O.  The  group 
OH  may  be  called  the  residue  of  water.  Special  names 
are  given  to  such  residues,  or  rests,  which,  as  we  shall 
see,  play  a  very  important  part,  as  the  formulae  of 
many  complicated  compounds  would  be  unintelligible 
without  them.  The  water  residue — that  is,  the  atomic 
complex  OH — is  called  hydroxyl  (from  the  Greek 
v&cop  =  water).  All  the  commoner  acids  and  bases,  with 
the  exception  of  hydrochloric,  hydrobromic,  hydriodic,  and 
hydrofluoric  acids,  contain  the  hydroxyl  group,  OH.  Nitric 
acid  contains  this  group  once,  sulphuric  acid  twice, 
as  we  saw  from  the  "  dissected "  formulae  of  these 
acids.  This  same  group  OH — hydroxyl — is  found  in 
cellulose,  CCH10O5,  which  behaves  towards  several  acids 


1  84      INTRODUCTION   TO   MODERN   CHEMISTRY. 

as  a  base.  One  molecule  of  caustic  potash,  K  —  OH, 
reacts  with  one  molecule  of  nitric  acid,  NO2  —  OH,  to 
form  the  salt  potassium  nitrate,  NO2  —  OK,  besides  a 
molecule  of  water  ;  and  one  molecule  of  sulphuric  acid, 

OH 
SO2  <^TT»    reacting    with    two    molecules    of    caustic 

potash,    2K—  OH,    forms  the   neutral   salt    SO2 

and  two  molecules  of  water.  The  reaction  of  cellulose 
with  nitric  acid  is  similar  to  these  ;  three  molecules  of 
nitric  acid  interact  with  the  three  hydroxyl  groups  in 
a  molecule  of  cellulose,  and  the  products  are  a  salt- 
like  compound  and  water.  The  following  equations 
express  these  reactions  :— 

(i)  K—  OH         +    NO2—  OH     =  NO2—  OK  +         H2O. 

One  molecule  caustic  +     nitric  acid      =    potassium     +    one  molecule 
potash  nitrate  water. 


Two  molecules        one  molecule  _  neutral  potas-         two  molecules 
caustic  potash        sulphuric  acid  ~~  sium  sulphate  water. 


(iii)  C6H7O2(OH>  +  sNO2—  OH  =  (NO2—  O)3C6H7O2  + 

One  molecule        three  mols.       one  molecule  gun-  three 

cellulose  nitric  acid  ~  cotton  mols.  water. 


The  compound  produced  by  the  action  of  nitric  acid 
on  cellulose  (in  practice  cotton  wool  is  employed)  would 
be  properly  called  cellulose  nitrate ;  but  it  is  generally 
known  as  nitrocellulose,  or,  in  ordinary  life,  as  gun- 
cotton. 

The  water  residue,  the  hydroxyl  group  OH,  also 
occurs  in  glycerin.  Glycerin  is  a  constituent  of  animal 
and  vegetable  fats,  all  of  which  are  glycerin  salts  of 


NITROGLYCERIN.  185 

fatty  acids.  In  these  fats  glycerin  behaves  towards 
the  fatty  acids  like  a  base.  Glycerin  also  reacts  with 
nitric  acid,  and,  like  cellulose,  with  three  molecules 
of  -that  acid.  The  formula  of  glycerin  is  C3H8O3 ; 
written  so  as  to  indicate  three  hydroxyl  groups,  this 
formula  becomes  C3H5(OH)3.  The  reaction  with  nitric 
acid  is  expressed  thus  by  an  equation : — 

C3H5(OH)3    +    3NO-OH   =  (NO-O)3C3H5  +        3H2O. 

One  molecule  three  mols.  one  molecule  three  mols. 

glycerin  nitric  acid  nitroglycerin  water. 

The  compound  which  is  thus  produced  is  an  oily 
liquid ;  it  might  be  called  glycerin  nitrate,  but  it  is 
usually  known  as  nitroglycerin.  The  names  nitro- 
glycerin and  nitrocellulose  are  derived  from  the  fact 
that  the  group  NO2,  which  is  the  rest  of  nitric  acid 
(NO2— OH),  is  called  the  nitrogroup. 

Forty  years  ago  it  was  supposed,  wrongly,  that  the  nitrogroup, 
NO.J,  entered  into  cellulose  and  glycerin  in  the  reactions  we 
have  just  considered;  hence  the  products  of  these  reactions  were 
called  nitrocellulose  and  nitroglycerin.  But  that  is  not  what 
really  occurs.  It  is  not  the  rest  NO.,  which  forms  a  part  of  the 
new  compounds ;  but  the  reactions  of  nitric  acid  with  cellulose 
and  glycerin  are  exactly  similar  to  the  reactions  of  that  acid  with 
bases  (such  as  potash)  whereby  salts  are  produced,  as  is  shown 
in  the  equations  already  given.  The  names  originally  given  to 
the  two  compounds  have,  however,  remained  in  ordinary  use. 
We  shall  learn  later  something  about  the  true  nitrocompounds. 

The  nitric  acid  used  for  making  guncotton  and  nitroglycerin 
must  be  as  free  from  water  as  possible.  The  equations  that  have 
been  given  show  that  three  molecules  of  water  are  produced  in 
the  preparation  of  these  compounds  (the  weight  of  water  formed 
can  be  calculated  easily) ;  hence  the  concentrated  nitric  acid 
used  gets  gradually  diluted  as  the  action  proceeds.  The  remedy 
for  this  is  found  by  allowing  a  mixture  of  nitric  and  sulphuric 


1 86      INTRODUCTION   TO   MODERN   CHEMISTRY. 

acids  to  act  on  cotton  wool  or  glycerin,  in  place  of  nitric  acid 
alone.  As  we  know  (see  p.  45),  sulphuric  acid  draws  water  to 
itself ;  it  seizes  hold  of  all  the  water  that  is  formed  in  the 
chemical  reaction  which  results  in  the  preparation  of  the  explosive 
substance.  The  addition  of  sulphuric  acid  insures  that  the 
nitric  acid  is  not  diluted  by  water  at  any  stage  of  the  process  of 
nitration^  as  the  action  is  called.  We  have  already  referred  to 
the  great  demand  for  sulphuric  acid  for  the  manufacture  of  nitro- 
glycerin  and  guncotton  (p.  157);  the  reason  for  that  demand  is 
now  apparent. 

Guncotton  and  nitroglycerin  are  the  main  foundations 
of  the  newer  powders  and  explosives.  The  empirical 
formulae  *  of  guncotton  and  nitroglycerin  are  C0H7N3On 
and  C3H5N3O9  respectively.  Let  us  suppose  these  com- 
pounds to  be  burning,  and  let  us  consider  what  will  be 
the  products  of  combustion  of  the  carbon,  hydrogen, 
nitrogen,  and  oxygen.  As  there  is  plenty  of  oxygen 
in  the  compounds,  the  carbon  will  be  burnt  to  carbon 
dioxide,  an  invisible  gas,  the  hydrogen  will  become 
water,  which  will  also  be  an  invisible  gas  at  the  high 
temperature  of  the  reaction,  and  the  nitrogen  will  be 
set  free,  also  as  an  invisible  gas.  In  a  word,  all  the 
products  of  the  explosion  of  either  of  the  two  sub- 
stances are  invisible  gases.  The  substances  burn, 
therefore,  without  smoke,  in  distinction  to  gunpowder, 
the  smoke  of  which  consists  (as  we  have  seen,  p.  181) 
of  potassium  salts.  The  application  of  these  two 
explosives  to  gunnery  has  led  to  the  preparation  of 
smokeless  powders. 

*  An  empirical  formula  states  the  total  number  of  atoms  of 
each  element  in  a  compound,  without  indicating  in  any  way  the 
supposed  arrangement  of  the  atoms.  [TR.] 


BLASTING   GELATIN   AND  DYNAMITE.  l8/ 

Guncotton  and  nitroglycerin  are  much  more  powerful 
explosives  than  the  old  powder.  That  powder  is  a 
mixture  of  substances,  and  it  burns  away,  as  other 
mixtures  do.  But  guncotton  and  nitroglycerin  are 
not  explosive  mixtures  of  substances ;  the  molecule 
of  each  itself  falls  to  pieces  when  explosion  occurs. 
Whereas  a  kilogram  of  the  old  powder  required,  perhaps, 
a  hundredth  of  a  second  for  its  combustion,  a  kilogram 
of  guncotton  is  decomposed  in  the  fifty  thousandth  of 
a  second ;  and  this  difference  of  itself  makes  the  gun- 
cotton  a  much  superior  explosive. 

We  know  that  nitroglycerin  is  a  liquid.  To  get  it 
into  the  form  of  cartridges,  for  use  as  an  explosive,  it 
has  long  been  customary  to  mix  the  liquid  with  a  fine 
sand  called  kieselguhr]  this  mixture  is  called  dynamite. 
The  explosive  force  of  the  nitroglycerin  is  tempered  by 
the  sand.  At  a  later  time  it  was  found  that  if  gun- 
cotton  is  mixed  with  nitroglycerin  the  mixture  sets  to 
a  jelly-like  mass  ;  in  this  case  the  explosive  force  of 
the  nitroglycerin  is  not  modified  by  sand,  but,  rather, 
it  is  added  to  that  of  the  guncotton.  There  are  many 
solvents  for  blasting  gelatin  (as  the  jelly-like  mixture 
of  guncotton  and  nitroglycerin  is  called) — for  instance, 
acetone,  a  compound  we  shall  meet  when  we  come  to 
the  compounds  of  carbon.  If  blasting  gelatin  diluted 
with  acetone  is  passed  between  rollers  placed  near 
together,  it  comes  out  in  the  form  of  sheets  ;  and  as  the 
acetone  evaporates  quickly,  the  explosive  is  obtained 
in  plates.  When  these  plates  are  cut  into  very  small 
pieces  they  form  the  modern  smokeless  powder.  The 
non-formation  of  smoke  has  been  explained  already; 


1 88      INTRODUCTION   TO   MODERN    CHEMISTRY. 

to  put  it  in  a  word,  it  is  due  to  the  absence  of  potassium 
salts  in  the  explosive.  The  results  of  using  smokeless 
powder  in  small  arms  are  extraordinary :  the  bullets 
are  propelled  for  more  than  three  kilometres  [over  one 
and  three-quarters  miles]. 

If  nitrate  of  potassium  or  nitrate  of  sodium  is  fused 
with  lead,  the  lead  is  oxidised,  by  the  oxygen  of  the 
nitrate,  to  lead  oxide,  PbO ;  and  the  nitrate,  giving  up 
an  atom  of  oxygen,  becomes  a  nitrite  (compare  p.  164). 

(i)        KN03       +      Pb       =  KNO2  +        PbO. 

Potassium  nitrate  -f-     lead      =    potassium  nitrite      +     lead  oxide. 

(ii)     NaNOa          4-      Pb       =  NaNO,          +         PbO. 

Sodium  nitrate      +       lead      =      sodium  nitrite       +     lead  oxide. 

The  salts  of  nitric  acid  are  called  nitrates  ;  the  salts 
of  nitrous  acid  are  called  nitrites.  Sodium  nitrite, 
which  is  cheaper  than  potassium  nitrite,  because  Chili 
saltpetre  is  cheaper  than  potash  saltpetre,  is  much  used 
in  the  manufacture  of  coal-tar  colours.  The  preparation 
of  this  salt  is  a  not  unimportant  branch  of  chemical 
industry. 


PHOSPHORUS. 
PHOSPHORUS  was  discovered  in  1670. 

Phosphorus  was  one  of  those  casual  results  which  accrued  to 
the  world  of  these  times  from  the  unmeaning  investigations  of 
the  alchemists.  A  Hamburg  merchant,  Brandt  by  name,  went 
to  the  goldmakers  to  repair  his  straitened  circumstances.  Every- 
one in  those  days  believed  in  transmutation,  as  the  process  of 
turning  the  base  metals  into  gold  and  silver  was  called.  This 
transmutation  was  to  be  effected  by  some  methods  and  tinctures, 
which,  however,  were  still  to  be  discovered.  The  important 
question  was,  who  should  be  the  first  to  find  the  stone  of 
wisdom  ?  Brandt  worked  for  a  long  time  before  he  came  to  the 
conclusion  that  nothing  was  to  be  gained  by  him  as  long  as  he 
followed  the  ordinary  methods.  At  last  he  hit  on  the  notion  of 
extracting  the  mysterious  principle  from  the  products  of  the 
living  organism,  and  he  thought  that  urine  would  be  the  most 
suitable  starting-point  for  his  investigations.  We  laugh  at  such 
a  conception  nowadays.  But  in  those  days,  when  no  accurate 
examinations  had  been  made  of  life-processes,  when  phrases 
and  empty  jingles  of  words  passed  for  learning,  and  faith  and 
intention  for  knowledge,  and  when  laws  were  made  before 
observations,  there  was  something  plausible,  especially  to  a 
dilettante  like  Brandt,  about  the  notion  that  man  was  the  most 
perfect  machine,  the  machine  in  which  all  substances  and  forces 
were  brought  to  the  highest  development  and  action.  What  was 
separated  and  disengaged  from  this  quintessence  of  creation,  this 
world  in  miniature,  from  the  microcosm,  that  must  be  the  most 
excellent  and  the  most  active  of  all  things.  Urine  was  evidently 
the  only  source  whence  the  stone  of  wisdom  could  be  obtained. 

189 


1 90      INTRODUCTION   TO   MODERN   CHEMISTRY. 

There  is  a  small  quantity  of  calcium  phosphate  in  urine ;  for 
albumin  contains  phosphorus  ;  and  the  albumin  that  is  used  in 
the  body  yields  phosphates  among  its  products  of  decomposition. 
All  such  decomposition-products  can  be  removed  from  the  body 
only  in  the  urine;  hence  the  existence  therein  of  calcium 
phosphate,  from  which  Brandt  prepared  phosphorus. 

Phosphorus  is  prepared  nowadays  from  bones,  be- 
cause these  are  rich  in  calcium  phosphate.  The 
phosphoric  acid  is  reduced  to  phosphorus  by  heating 
strongly  with  charcoal  in  retorts,  from  which  the  phos- 
phorus distils  over.  Phosphorus  is  a 
wax-like  body  of  a  pale  yellow  colour. 
Its  most  striking  property  is  its  extra- 
ordinary inflammability — that  is,  great 
readiness  to  combine  with  oxygen — 
which  makes  it  possible  to  raise 
phosphorus  to  the  ignition  point  with 
the  greatest  ease.  Phosphorus  cannot 
be  left  in  the  air  without  at  once  be- 
ginning to  oxidise ;  hence  it  must  be 

Fig.  47- Phosphorus     kept    under    water    /see    fig     ^  .    pre. 
kept  under  water. 

served  thus  from  contact  with  the 
air,  it  remains  unchanged  for  any  length  of  time. 
The  name  phosphorus  is  derived  from  the  Greek, 
and  means  light-bearer.  The  name  was  given  because, 
when  exposed  to  the  air,  the  substance  glows  in  the 
dark,  owing  to  the  slow  oxidation  that  goes  on :  the 
glowing  is  so  slight  that  it  is  not  visible  in  daylight. 
Slight  rubbing,  which,  like  all  friction,  raises  the 
temperature,  soon  causes  ignition — that  is  to  say, 
changes  the  slow  oxidation  into  rapid  oxidation,  accom- 
panied by  the  phenomena  of  fire.  We  have  already 


PHOSPHORUS   MATCHES.  191 

more  than  once  ignited  phosphorus,  and  seen  it  burn 
to  a  white  smoke,  which  is  an  oxide  of  phosphorus. 
That  oxide  is  composed  of  two  atoms  of  phosphorus 
and  five  atoms  of  oxygen  ;  its  formula  is,  therefore, 
P2O5.  It  is  called  either  phosphorus  pentoxide  or 
phosphoric  anhydride,  as  we  already  know  (p.  117). 

The  ready  inflammability  of  phosphorus  has  been 
known,  of  course,  since  the  substance  was  discovered  ; 
nevertheless,  a  long  time  passed  before  a  convenient 
method  was  found  for  using  this  property  to  obtain 
fire.  It  was  not  till  the  thirties  of  the  nineteenth 
century — that  is,  not  till  nearly  two  hundred  years 
after  the  discovery  of  phosphorus — that  phosphorus 
matches  were  made,  which  at  last  enabled  everyone 
to  obtain  fire  without  trouble.  We  cannot  now  pro- 
perly picture  to  ourselves  a  time  when  there  were  no 
matches.  At  first  some  phosphorus  was  fastened  to 
little  slips  of  wood  which  had  been  dipped  in  sulphur. 
The  small  quantity  of  phosphorus  was  ignited  by 
rubbing  ;  the  flame  passed  to  the  easily  ignited  sulphur, 
which  in  turn  set  fire  to  the  wood. 

A  sad  trouble  accompanied  the  manufacture  of  these 
phosphorus  matches  ;  the  workpeople  became  gradually 
seriously  diseased  from  constantly  handling  the  poisonous 
yellow  phosphorus.  A  remedy  was  urgently  called  for. 
The  discovery  of  red  phosphorus  made  a  remedy  pos- 
sible. The  conditions  are  these :  if  ordinary  yellow 
phosphorus  is  heated  for  a  considerable  time  to  250°, 
it  is  gradually  changed  into  a  red  powder  which  is  not 
poisonous. 


INTRODUCTION    TO   MODERN    CHEMISTRY. 


We  can  bring  about  this  change  in  the  manner 
represented  in  fig.  48.  A  small  piece  of  ordinary 
yellow,  easily  inflammable  phosphorus  has  been  placed 
in  the  inner  tube  A,  which  has  then  been  closed  by 
fusing  the  glass  and  allowing  it  to  run  together.  The 

quantity  of  air  in 
the  tube  is  so  small 
that  the  phosphorus 
is  practically  cut  off 
from  the  air ;  hence 
the  phosphorus  can- 
not burn  when  it  is 
heated,  for  lack  of 
oxygen.  If  the  tube 
with  the  phosphorus 
in  it  were  heated 
directly  over  a  flame, 
the  tube  would  very 
probably  break ;  and 
if  that  happened, 
the  experimenter 
would  be  exposed 
to  the  danger  of 
being  very  badly 
burnt  by  some  of 
the  glowing  pieces 
of  phosphorus  which  would  be  scattered  in  all  direc- 
tions. It  is  less  dangerous  to  heat  the  tube  in  a  bath 
kept  at  the  proper  temperature.  For  this  purpose  we 
shake  some  phenanthrene  into  the  outer,  wide  tube  B — 
phenanthrene  is  a  compound  obtained  from  coal-tar, 
which  melts  easily  and  boils  at  360°  C.  [680°  F.]— and 


Fig.  48.— Change  of  yellow  into  red 
phosphorus. 


SAFETY   MATCHES.  193 

we  then  cause  the  phenanthrene  to  boil  by  means  of 
the  flame  placed  below  the  tube.  The  phosphorus 
which  is  surrounded  by  the  vapour  of  phenanthrene, 
and  is  kept  at  360°  C,  soon  begins  to  be  coloured 
red,  for  the  change  proceeds  very  quickly  at  this  high 
temperature. 

The  red  phosphorus  thus  produced  is  not  poisonous, 
and  it  is  also  much  less  inflammable  than  yellow  phos- 
phorus. It  cannot  be  ignited  by  mere  rubbing;  on 
the  contrary,  it  takes  fire  only  when  it  is  rubbed  with 
substances  which  are  very  rich  in  oxygen.  The  pre- 
paration of  "  safety  matches  "  is  based  on  this  property 
of  red  phosphorus.  The  heads  of  these  matches  con- 
tain no  phosphorus  (in  contradistinction  to  the  older 
phosphorus  matches),  but  are  composed  of  substances 
very  rich  in  oxygen,  such  as  potassium  chlorate,  KC1O3, 
potassium  bichromate,  K2Cr2O7,  and  lead  peroxide,  PbO2, 
wherewith  lead  sulphide,  PbS,  is  often  mixed,  because 
this  substance  has  been  found  very  suitable.  The 
rubbing  surface  whereon  the  safety  matches  are  ignited 
consists  of  red  phosphorus,  made  to  adhere  to  the  box 
by  means  of  glue. 

A  third  modification  of  phosphorus  has  been  obtained. 
If  the  tube  wherein  we  prepared  red  phosphorus  had 
been  heated,  not  to  360°  only,  but  to  a  temperature  as 
high  as  530°,  crystals  of  phosphorus  would  have  sub- 
limed in  the  upper,  and  therefore  cooler,  part  of  the 
tube.  These  crystals  form  a  third  variety  of  phos- 
pho/us  ;  they  are  not  so  easily  inflammable  as  ordinary 
phosphorus,  and  are  less  active  than  that  substance. 

13 


194     INTRODUCTION   TO   MODERN   CHEMISTRY. 


THE   VARIOUS  MODIFICATIONS  OF   CERTAIN    ELEMENTS. 

We  have  seen  that  phosphorus  exists  in  more  than 
one  form,  or  modification.  Several  other  elements  are 
like  phosphorus  in  this  respect.  Such  a  behaviour  of 
an  element  seems  at  first  sight  unthinkable ;  it  cannot 
be  understood  ofF-hand  :  for  an  element — a  something 
which  exists,  so  to  speak,  alone  and  by  itself,  as  a 
thing  in  itself — can  surely  have  but  one  kind  of  qualities, 
can  surely  exist  only  in  one  form,  which  cannot  vary. 

We  cannot  suppose  that  there  are  several  kinds  of 
gold,  several  kinds  of  lead,  or  several  kinds  of  sodium. 
Chemically  pure  gold — the  element  gold — is  once  for 
all  gold,  exhibits  now  and  always  the  same  chemical 
behaviour,  etc.  This  apparently  incontestable  demand 
of  our  perceptive  faculties  seems  to  be  contradicted  by 
what  we  have  found  to  be  the  facts  regarding  phosphorus. 
The  apparent  contradiction  can  be  easily  removed, 
however,  by  the  aid  of  the  atomic  theory,  and  an 
elucidation  of  the  facts  can  be  arrived  at  in  the  follow- 
ing way.  The  atomic  weight  of  phosphorus  is  31,  as 
determined  by  the  analysis  of  phosphorus  pentachloride, 
for  example.  As  ordinary  phosphorus  is  easily  gasified, 
there  is  no  difficulty  in  determining  the  specific  gravity, 
and  hence  the  molecular  weight,  of  phosphorus  gas 
(see  p.  ill).  The  specific  gravity  of  gaseous  phosphorus 
is  sixty-two  times  greater  than  that  of  hydrogen  at  the 
same  temperature.  For  the  reasons  laid  down  in  the 
page  already  referred  to,  .it  follows  that  the  molecular 
weight  of  phosphorus  gas  is  124,  which  is  four  times 
31 — four  times  the  atomic  weight  of  phosphorus.  Hence 


TWO   VARIETIES   OF   PHOSPHORUS.  195 

the  molecule  of  [the  gaseous  phosphorus  obtained  by 
heating]  ordinary  phosphorus  consists  of  four  atoms ; 
the  molecule  of  red  phosphorus  may  consist  of  [a 
number  of  atoms  different  from  the  number  whereof 
the  molecule  of  ordinary  phosphorus  is  composed, 
say]  perhaps  twenty  atoms ;  and  the  molecule  of  the 
crystalline  phosphorus  produced  at  a  high  temperature 
may  perhaps  consist  of,  say,  forty  atoms. 


We  can  only  make  an  approximate  guess  at  the  number  of 
atoms  in  the  molecules  of  red  and  crystalline  phosphorus ;  for 
we  have  at  present  no  method  for  finding  the  molecular  weights 
of  substances  which  cannot  be  gasified  and  are  insoluble  in  all 
menstrua.  The  two  kinds  of  phosphorus — red  and  crystalline — 
are  insoluble  in  all  menstrua,  nor  can  they  be  gasified  as  such. 

We  must  not  pass  over  the  fact  that  methods  have  been 
developed,  in  the  last  ten  years,  for  finding  the  molecular  weights 
of  bodies  which  cannot  be  gasified,  from  the  behaviour  of  these 
bodies  in  solutions.  The  principles  of  the  methods  are,  briefly, 
these.  Everyone  .knows  that  water  freezes  at  o°  C.  [32°  F.]. 
Now,  if  we  dissolve  common  salt  in  water,  we  obtain  a  solution 
which  freezes  considerably  under  o° ;  a  saturated  solution  of  salt, 
for  instance,  freezes  at  -  20*37°  C.  [  -  4*66°  F.].  If  such  lowerings 
of  freezing  points  are  examined  more  closely,  it  is  found  that 
they  are  not  chance  occurrences ;  it  is  found  that,  if  a  weighed 
quantity  of  a  substance  is  dissolved  in  a  weighed  quantity  of  any 
solvent,  the  freezing  point  of  the  solvent  is  not  only  lower  than 
that  of  the  pure  solvent,  but  that  the  lowering  bears  a  perfectly 
definite  relation  to  the  freezing  point  of  the  pure  solvent.  Experi- 
ments have  shown  that  the  molecular  weight  of  the  body  in 
solution  can  be  calculated  from  the  observed  lowering  of  the 
freezing  point  of  the  solvent.  It  has  also  been  found  that,  when 
a  weighed  quantity  of  a  body  is  dissolved  in  a  weighed  quantity 
of  a  solvent— let  us  say  water — and  the  boiling  point  of  the 
solution  is  determined,  this  boiling  point  is  higher  than  that  of 
the  pure  solvent— in  this  case  is  higher  than  100°  C.  [212°  F.]. 
The  boiling  point  of  a  saturated  solution  of  common  salt,  for 


196      INTRODUCTION   TO   MODERN    CHEMISTRY. 

instance,  is  109-25°  C.  [228-65°  F.].  The  molecular  weight  of  the 
body  in  solution  can  be  calculated  from  the  observed  raising  of 
the  boiling  point  of  the  solvent. 

These  methods  for  determining  molecular  weights  give  results 
which  agree  well  and  are  trustworthy  only  when  very  dilute 
solutions  are  used ;  for  example,  solutions  containing  about 
•3  gram  of  substance  in  40  grams  of  solvent.  The  depression 
of  freezing  point  or  increase  of  boiling  point  does  not  amount 
to  more  than  a  few  tenths  of  a  degree  in  such  solutions ;  hence 
it  is  necessary  to  work  with  very  delicate  thermometers  and  in 
specially  constructed  apparatus,  protected,  for  instance,  from  all 
air-draughts.  We  see  that  the  very  remarkable  law  which  states 
that  equal  volumes  of  all  gases  contain  equal  numbers  of 
molecules  finds  its  counterpart  in  the  influence  of  any  dissolved 
body  on  its  solvent  which  can  be  measured  once  for  all,  and 
serves  as  the  basis  of  a  method  of  calculating  the  weights  of 
molecules. 


We  have  only  appraised  the  number  of  atoms  in  the 
molecules  of  red  and  crystalline  phosphorus,  as  con- 
trasted with  the  four  atoms  whereof  the  molecule  of 
yellow  phosphorus  is  formed.  At  any  rate,  it  is  easily 
seen  that  the  appearance  of  an  element  in  several 
modifications  finds  an  explanation  in  terms  of  the  fact, 
established  experimentally,  that  the  molecule  of  an 
element  may  consist  sometimes  of  a  greater  and  some- 
times of  a  smaller  number  of  atoms,  according  to  the 
conditions  of  preparation  of  the  element. 

One  can  see  at  once  that  there  is  nothing  peculiar 
about  this  explanation,  nothing  which  clashes  with  our 
general  conception  of  an  element.  Rather  it  is  the 
other  way  round,  as  one  sees  after  a  little  careful 
attention ;  for  without  the  conception  of  the  atomic 
condition  of  the  elements,  and  the  possibility  (con- 
nected therewith)  of  elementary  molecules  being  some- 


OZONE.  197 

times  larger  and  sometimes  smaller,  the  existence  of 
elements  in  different  modifications,  which  are  now 
greater,  now  smaller,  atomic  complexes,  would  be 
quite  inexplicable.  This  appearance  of  the  elements 
in  various  modifications  is,  indeed,  a  support  of  the 
atomic  hypothesis,  for  this  hypothesis  is  alone  able 
to  explain  it. 

OZONE. 

We  are  now  in  a  position  to  give  an  explanation  of 
ozone. 

Ozone  is  a  modification  of  oxygen.  While  the 
ordinary  oxygen  that  is  found  in  the  atmosphere  con- 
sists of  molecules  of  the  composition  O2  (see  p.  Hi), 
ozone  consists  of  molecules  of  the  composition  O3, 
molecules  composed  of  three  atoms.  Ozone  is  most 
conveniently  obtained  by  passing  electric  sparks  through 
oxygen  ;  the  great  shock  changes  three  molecules  of 
ordinary  oxygen,  O2,  into  two  molecules  of  ozone,  O3. 
Ozone  has  a  peculiar  smell.  This  smell  can  sometimes 
be  perceived  after  a  severe  thunderstorm,  wherein  the 
lightning  has  brought  about  the  formation  of  ozone. 
Ozone  soon  changes  back  to  ordinary  oxygen. 


PHOSPHORETTED  HYDROGEN. 

Of  the  three  compounds  of  phosphorus  and  hydrogen 
which  are  known — PH3,  P2H4,  and  P4H2— we  shall 
consider  only  one,  namely,  the  compound  PH3,  the 
composition  of  which  corresponds  with  that  of  ammonia, 


198      INTRODUCTION    TO   MODERN    CHEMISTRY. 

NH3.  Like  ammonia,  this  phosphoretted  hydrogen  is 
a  gas.  The  gas  is  spontaneously  inflammable  ;  that  is 
to  say,  it  takes  fire  as  soon  as  it  comes  into  contact  with 
the  air.  We  shall,  therefore,  prepare  this  compound. 
The  inflammability  of  this  gas  is  due  to  the  rapid  union 
of  its  phosphorus  and  its  hydrogen  with  the  oxygen  of 
the  air  to  form  phosphorus  pentoxide,  P2O;-,,  and  water, 
H2O,  respectively. 


2PH3  +         8O  P2O5  + 

Phosphoretted        +        oxygen          •       phosphorus       +       water. 
hydrogen  (from  the  air)  pentoxide 

This  phosphoretted  hydrogen  is  obtained  by  boiling 
caustic  potash  solution  with  phosphorus. 

The  products  of  this  reaction  are  phosphoretted  hydrogen  and 
a  salt  called  potassium  hypophosphite  :  — 

4P       +       3KOH        +3H20=  PH3        +  3P02H2K. 

Phosphorus  +  caustic  potash  +  water  =  phosphoretted  +  potassium 

solution  hydrogen  gas    hypophosphite. 

Were  we  to  put  phosphorus  in  caustic  potash  solu- 
tion contained  in  the  retort  A  of  the  apparatus  shown 
in  fig.  49,  and  to  warm,  we  should  have  an  explosion. 
Phosphoretted  hydrogen  so  readily  causes  explosions 
that  an  unskilled  person  had  much  better  not  attempt 
to  prepare  it.  The  phosphoretted  hydrogen  that  formed 
in  the  retort  would  at  once  react  with  the  air  therein 
with  explosive  violence,  and  the  retort  would  be 
shattered.  To  prevent  this  mishap  we  fill  the  appa- 
ratus, at  the  beginning  of  the  experiment,  with  a  gas 
wherewith  phosphoretted  hydrogen  does  not  react  — 
with  hydrogen  gas,  for  instance.  This  is  done,  in  the 


PHOSPHORETTED   HYDROGEN. 


199 


way  shown  in  fig.  49,  by  connecting  the  retort  with  a 
Kipp's  apparatus  for  generating  hydrogen,  and  allowing 
the  hydrogen  to  flow  through  the  apparatus.  As  soon 
as  the  whole  of  the  air  has  been  driven  out  of  the 
apparatus,  the  potash  solution  may  be  warmed  without 
danger.  The  phosphoretted  hydrogen  passes  through 
the  tube  B  into  water,  and  rises  through  the  water  in 


Fig.  49. — Preparation  of  spontaneously  inflammable  phosphoretted  hydrogen. 

bubbles.  Each  bubble  takes  fire  as  it  comes  into  con- 
tact with  the  air  at  the  surface  of  the  water,  and  the 
white  smoke  of  phosphorus  pentoxide  which  is  formed 
ascends  in  the  form  of  a  ring.  It  looks  almost  as  if 
the  water  had  caught  fire  by  chance. 

Phosphorus,  like  sulphur,  combines  directly  with 
chlorine,  bromine,  and  iodine.  Phosphorus  pentachloride, 
PC16,  is  much  used  in  the  laboratory,  because  it  enables 


200      INTRODUCTION   TO   MODERN    CHEMISTRY. 

us  to  effect  the  replacement  of  the  hydroxyl  group  (see 
p.  183)  by  chlorine  in  the  most  different  kinds  of  com- 
pounds. Phosphorus  pentachloride  is  prepared  by 
passing  chlorine  gas  over  phosphorus  gently  warmed 
in  a  retort  :  — 

P  +          sCl  PC15. 

Phosphorus     +       chlorine  gas     =      phosphorus  pentachloride. 

If  we  wished  to  replace  one  hydroxyl  group  (one  OH) 
in  sulphuric  acid  by  an  atom  of  chlorine,  we  would 
allow  one  molecule  of  PC15  to  react  with  one  molecule 
of  sulphuric  acid,  H2SO4.  As  P  =  31  and  Cl  =  35-5, 


it  follows  that  +  ^  x535'5)  =  2°8'5  parts  by  weight 
are  equivalent  to  one  molecule  of  phosphorus  penta- 
chloride; and  as  H  =  i,  S  =  32,  and  O  =  16,  it 
follows  that  (2  H«  I}  +  S  +  (4  O'  I6)  =  98  parts  by 
weight  are  equivalent  to  one  molecule  of  sulphuric  acid. 

SO*<OH  +       PC1>    =    S°2<OH  +      POC13     +      HCL 

one  mol.  sul-  one  mo1*     _      chlorosul-  phosphorus        hydrochloric 

phuric  acid  +    Pn°sphorus        phonic  acid     +    oxychloride    "*         acid. 
pentachloride 

98  parts  208  's  parts          n6'5  parts  153  '5  parts          36*5  parts 

by  weight  by  weight  by  weight*  by  weight          by  weight. 

The  equation  shows  that  phosphorus  oxychloride 
and  hydrochloric  acid  are  obtained  in  the  reaction, 
besides  chlorosulphonic  acid  ;  these  substances  must 
be  separated  from  one  another.  As  there  is  no  water 
present,  gaseous  hydrochloric  acid  will  be  produced. 
If  the  operation  is  conducted  in  a  draught-chamber, 
the  acid  gas  will  be  carried  away  through  the  flue  into 


FRACTIONAL   DISTILLATION.  2OI 

the  open  air,  and  will  thus  be  got  rid  of.  In  order  to 
separate  the  chlorosulphonic  acid  and  the  phosphorus 
oxychloride,  both  of  which  are  liquids  that  can  be  dis- 
tilled, we  make  use  of  the  process  called  "fractional 
distillation"  That  process  is  conducted  with  such  an 
apparatus  as  is  represented  in  fig.  5  (p.  7).  Phos- 
phorus oxychloride  boils  at  107°  C.  [224*6°  F.],  and 
chlorosulphonic  acid  at  I58°C.  [316-4°  F.].  As  long  as 
phosphorus  oxychloride  is  distilling  over,  the  ther- 
mometer will  not  rise  above  107° ;  when  that  com- 
pound has  been  removed,  the  thermometer  will  rise  to 
158°  :  hence  it  is  only  necessary  to  collect  separately 
the  liquid  that  distils  over  at  158°  in  order  to  obtain 
chlorosulphonic  acid.  Substances  that  can  be  distilled 
are  separated  from  one  another  by  distillation,  using  a 
thermometer — that  is,  by  taking  advantage  of  the  differ- 
ences between  their  boiling  points :  solid  bodies  are 
purified,  as  we  know,  by  re-crystallisation  (see  p.  55)- 

In  phosphorus  pentachloride  we  have  a  means  for 
replacing  an  atomic  complex — in  this  case  the  hydroxyl 
group — by  an  atom  of  chlorine.  This  is  the  first  ex- 
ample we  have  had  of  the  possibility  of  replacing,  not 
only  single  atoms,  but  atomic  groups,  in  compounds, 
by  other  atoms,  according  to  definite  rules.  Phos- 
phorus pentachloride  is  a  "group  re-agent" 

There  are  two  oxides  of  phosphorus  ;  two  atoms 
of  phosphorus  combine  with  three  atoms  of  oxygen, 
forming  the  trioxide  P2O3,  and  also  with  five  atoms 
of  oxygen,  forming  the  pentoxide  P2O5,  which  has  been 
so  often  mentioned  already.  Both  of  these  oxides 


202      INTRODUCTION    TO   MODERN    CHEMISTRY. 

react  with  water  to  form  acids  :  but  we  shall  consider 
only  the  phosphoric  acids  derived  from  the  pentoxide  ; 
phosphorous  acid,  which  is  derived  from  phosphorus 
trioxide  (or  phosphorous  anhydride),  is  of  little  interest 
to  us.*  Phosphorus  pentoxide  yields  three  acids — 
called  metaphosphoric  acid,  pyrophosphoric  acid,  and 
orthophosphoric  acid — by  combining  with  one,  two,  or 
three  molecules  of  water.  Orthophosphoric  acid  is  by 
far  the  most  important  of  these  acids :  it  is  also  called 
ordinary  phosphoric  acid ;  and  when  phosphoric  acid  is 
spoken  of  without  any  qualifying  word,  it  is  always  this 
acid  that  is  meant. 

P205         +  H20  -     H2P206  (or,  halved,  HPO3). 

Phosphoric     +      one  molecule     =     metaphosphoric  acid, 
anhydride  water 

P205         +          2H20          =     H4P207. 
Phosphoric      +    two  molecules    =     pyrophosphoric  acid.f 
anhydride  water 

PA         +         3H20  =     H6P208  (or,  halved,  HSPO4). 

Phosphoric     +   three  molecules  =     orthophosphoric  acid, 
anhydride  water 

Orthophosphoric  acid,  or  simply  phosphoric  acid, 
has  the  formula  H3PO4 ;  for  it  is  not  necessary  to 
write  the  formula  as  H6P2O8 — that  is  used  only  to 
make  perfectly  clear  the  formation  of  the  acid  from 
phosphoric  anhydride.  The  simpler  formula,  H3PO4, 

*  There  is  a  third  oxide  of  phosphorus,  P2O4.  A  solution  of 
this  oxide  in  water  contains  both  phosphorous  acid  (H3PO3)  and 
phosphoric  acid  (H3PO4).  [TR.]. 

f  This  formula  cannot  be  halved  ;  the  half  formula  would 
contain  a"  half  atom  of  oxygen. 


BASICITY    OF   ACIDS.  203 

shows  the  proportion  wherein  the  phosphorus,  hydro- 
gen, and  oxygen  are  combined.  The  three  atoms 
of  hydrogen  are  replaceable  by  metal;  the  acid  is 
tribasic  (as  the  expression  is).  According  as  one,  two, 
or  three  atoms  of  hydrogen  are  replaced  by  a  metal, 
very  different  salts  are  obtained.  The  basicity  of  an 
oxy-acid  [very  often]  agrees  with  the  number  of 
hydroxyl  groups  in  the  acid  ;  for  it  is  [generally]  the 
hydrogen  of  the  hydroxyl  groups  that  is  replaceable 
by  metal.  Nitric  acid,  NO2 — OH,  is  monobasic; 

OH 
sulphuric  acid,  SO  "XQTT,  is  dibasic;    and   phosphoric 

/OH 
acid  is  tribasic,    PO— OH.     A  similar  state  of  affairs 

XOH 

holds  good  for  [i  any]  bases  :  the  number  of  hydroxyl 
groups  in  a  base  aeasures  what  is  called  the  "  acidity  " 
of  that  base.* 

We  are  acquainted  with  mono-acid  bases,  such  as 
caustic  potash,  K — OH,  and  caustic  soda,  Na — OH  ; 
and  there  are  also  bases  which  are  di-acid,  tri-acid, 
etc.  It  is  not  necessary  that  all  the  replaceable 
atoms  of  hydrogen  in  a  polybasic  acid  should  be 
replaced  by  the  same  metal  (or  by  the  ammonium 
group,  see  p.  i/i);  but  the  atoms  of  hydrogen  may 
be  replaced  by  different  metals  (compare  p.  159).  The 
metal  magnesium,  for  instance,  always  replaces  two 
atoms  of  hydrogen ;  hence,  if  we  replace  two  atoms 
of  hydrogen  in  phosphoric  acid  by  magnesium,  we 

*  Of  course,  it  is  ridiculous  to  talk  of  the  basicity  of  an  add 
and  the  acidity  of  a  base.  Most  unfortunately,  these  expressions 
have  been  in  common  use  for  many  years.  [TR.] 


204      INTRODUCTION   TO   MODERN   CHEMISTRY. 

get    a    salt    which    has    the    formula    MgHPO4,    or, 

c  ..       r>/^/Vk^Mg.      This  salt  is  found 
written   more   fully,    PO^-Cr 

X)H 

in  urine.  When  urine  putrefies,  ammonia  is  formed 
therein  ;  and  this  reacts  with  the  magnesium  salt,  and 
replaces  the  atom  of  hydrogen  by  ammonium,  NH4. 
Stale  urine,  then,  always  contains  ammonium  mag- 
nesium phosphate,  MgNH4PO4,  or,  written  more  fully, 


—  NH4. 


THE  BUILDING  UP  OF  PLANTS  FROM  INORGANIC 
SUBSTANCES. 

Now  that  we  are  acquainted  with  phosphoric  acid,  we 
know  something  of  all  the  inorganic  substances  and 
compounds  that  are  required  for  the  growth  of  plants — 
of  all  with  the  help  whereof  plants  live.  Plants  live  on 
inorganic  substances — on  the  constituents  of  non-living 
materials ;  whereas  the  animal  world  cannot  use  such 
material  for  their  growth,  but  can  live,  directly  or  in- 
directly, only  on  the  substances  that  are  produced  by 
plants.  For  when  we  eat  flesh,  etc.,  it  has  always  come 
from  animals  which  have  been  nourished  on  plants.  A 
survey  of  the  nutrition  of  plants,  at  this  point  in  our 
course,  besides  being  interesting  in  itself,  will  afford 
an  opportunity  for  the  elucidation  of  some  occurrences 


NUTRITION   OF   PLANTS.  20$ 

which  are  important  from  a  purely  chemical  point  of 
view. 

The  connection  between  the  plant  world  and  non- 
living matter — first  enunciated  by  Liebig — may  be  set 
forth  briefly  somewhat  as  follows.  If  a  plant,  or  a  part 
of  a  plant — wood,  for  instance — is  burnt,  ashes  remain. 
These  ashes,  which  were  formerly  thought  to  be  merely 
chance  constituents  of  the  plant,  represent  what  the 
plant  took  by  its  roots  from  the  soil  wherein  it  grew,  as 
being  absolutely  necessary  to  its  existence. 

If  a  soil  does  not  contain  the  main  constituents  which 
an  analysis  of  such  plant-ash  shows  to  be  contained 
therein,  the  plant  in  question  cannot  thrive  in  that  soil. 
The  ashes  of  different  plants  are  not  of  the  same  com- 
position; hence  one  plant  may  grow  and  thrive  on  a 
plot  whereon  another  plant  does  not  grow  or  grows 
badly.  Every  crop  raised  on  a  field  withdraws  from 
the  soil  of  the  field  a  considerable  number  of  kilos,  of 
inorganic  salts.  Since  Liebig  made  this  clear,  people 
have  realised  that  these  inorganic  salts  must  be  given 
back  to  the  soil.  This  is  done  by  using  artificial 
manures.  Strange  though  it  sounds,  nevertheless  it  is 
the  case,  that  all  agriculture  was  more  or  less  a  robbing 
of  the  soil  before  the  true  state  of  affairs  was  made 
known  by  Liebig ;  for  the  inorganic  constituents  of  the 
soil  that  were  carried  away  in  the  marketable  products 
of  agriculture  were  not  given  back  to  the  soil.  The 
stable  manure,  and  the  like,  that  were  put  on  the  soil 
carried  back  thereto  much  smaller  quantities  of  in- 
organic salts  than  were  sold  in  the  form  of  agricultural 
produce. 


206      INTRODUCTION    TO    MODERN    CHEMISTRY. 


The  following  table  presents  analyses  of  the  ashes  of 
some  products  of  the  vegetable  kingdom  : — 


Ash  of 
rye  grains. 

Ash  of 
barley  grains. 

Ash  of 
potatoes. 

Phosphoric  acid*  P2O5 

47-52  per  cent. 

38  8  per  cent. 

17-4  percent. 

Potash 

K?0 

34-50       » 

2I'I         „ 

60-4 

Magnesia 

MgO 

11-38 

7'0        „ 

47 

Lime  . 

CaO 

275 

17         » 

2-4 

Silicic  acid* 

Si02 

275 

29*3              M 

2'I 

Soda   . 

Na.,O 

0-52 

2-6 

Iron  oxide 

Fe203 

O'2O 

2'I         „ 

T2 

Chlorine 

Cl 

0-38 

— 

30 

Sulphuric  acid*  .  SO3 

— 

— 

6-2 

I  OO'O 

I  OO'O 

100-0 

These  analyses  exhibit  the  requirements  of  certain 
plants  as  regards  various  inorganic  compounds.  Every 
soil  suited  for  agriculture  contains  in  itself  most  of 
these  compounds.  Experience  shows  that  phosphoric 
acid  and  potash  salts  are  almost  the  only  compounds, 
of  those  found  in  plant  ashes,  which  must  be  supplied 
to  such  soils  in  order  to  maintain  their  fertility,  or  to 
bring  them  to  a  higher  degree  of  productiveness  by 
increasing  in  them  the  quantities  of  the  inorganic  com- 
pounds which  are  absolutely  necessary  for  plant  growth. 

Many   sources    of  phosphoric    acid    are   within   our 

*  The  names  phosphoric  acid,  silicic  acid,  and  sulphuric  acid 
are  here  applied  to  the  anhydrides  of  these  acids ;  and  the 
percentage  amounts  of  those  acids  are  calculated  as  these 
anhydrides.  This  is  a  custom  which  has  come  from  the  older 
analyses  of  plant-ashes,  and  is  retained  in  this  department 
of  analysis. 


MAKING   SUPERPHOSPHATE.  2O/ 

reach.  Besides  bones,  which  we  already  know  (p.  190) 
to  consist  for  the  most  part  of  calcium  phosphate,  there 
is  the  mineral  phosphorite,  which  is  more  or  less  pure 
calcium  phosphate ;  and  we  could  hardly  even  enumer- 
ate the  less  important  sources  of  this  compound. 

The  calcium  phosphate  in  bones  and  in  phosphorite 
is  quite  insoluble  in  water ;  but  the  roots  of  plants  are 
able  to  absorb  only  bodies  that  are  dissolved  by  water. 
By  the  process  called  disintegration  nature  provides 
for  the  rendering  soluble  of  those  rock-fragments  which 
are  present  in  every  soil  and  represent  the  supply  of 
calcium  phosphate  insoluble  in  water.  That  process  is 
somewhat  as  follows.  The  carbonic  acid  which  circu- 
lates through  the  soil  with  the  air — for  carbonic  acid  is 
a  constituent  of  the  air — acts,  along  with  the  moisture 
in  the  soil,  on  the  rock-fragments,  and,  attacking  the 
insoluble  calcium  phosphate  therein,  converts  it  into  a 
calcium  salt  of  phosphoric  acid  which  is  soluble  in 
water. 

This  natural  process  takes  place  very  slowly  in 
soils.  If  a  soil  is  to  be  manured  with  the  calcium 
phosphate  of  bones,  or  phosphorite,  that  salt  must  be 
changed  to  a  soluble  calcium  phosphate  before  it  is  put 
into  the  soil.  The  roots  absorb  this  phosphate  dissolved 
in  the  moisture  of  the  soil,  and  the  good  results  of  the 
manuring  are  apparent  in  the  next  crop  that  is  taken 
from  the  soil. 

This  change  of  one  calcium  phosphate  into  another 
which  is  soluble  in  water  is  known  as  making  "super- 
phosphate "  ;  the  phosphoric  acid  in  the  product  is  in 
a  condition  wherein  it  is  more  valuable  to  the  agri- 
culturists than  when  it  is  in  the  raw  material. 


208      INTRODUCTION    TO   MODERN    CHEMISTRY. 

The  production  of  soluble  phosphate  can  be  accom- 
plished only  by  chemical  methods.  The  finely  powdered 
raw  material  is  covered  with  sulphuric  acid,  which, 
being  the  stronger  acid,  seizes  the  calcium,  and, 
replacing  part  of  the  phosphoric  acid,  causes  the 
formation  of  a  calcium  phosphate  that  is  soluble  in 
water  and  is,  therefore,  fitted  for  absorption  by  the 
roots  of  the  plants. 

The  following  considerations  will  enable  us  to  follow 
this  process,  and  will  also  give  us  an  opportunity  of 
examining  more  fully  than  before  the  conception  of  the 
basicity  of  an  acid.  Metal  can  take  the  place  of  hydro- 
gen atoms  in  phosphoric  acid  (H3PO4),  as  in  all 
other  acids.  Sodium,  for  instance,  produces  the  salt 
Na3PO4,  sodium  phosphate.  As  the  three  atoms  of 
hydrogen  in  phosphoric  acid  are  all  replaceable,  the 
acid  is  said  to  be  tribasic.  Now,  the  metal  calcium 
does  not,  like  sodium,  replace  one  atom  of  hydrogen 
from  an  acid,  but  always  two  atoms  of  hydrogen. 
Hence  the  formula  of  the  calcium  salt  of  the  tribasic 
phosphoric  acid  must  be  Ca3(PO4)2 ;  for  three  atoms  of 
calcium  correspond  to  six  atoms  of  hydrogen,  and  two 
molecules  of  phosphoric  acid — two  molecules  of  H3PO4 
— are  required  to  yield  six  atoms  of  hydrogen.  As 
calcium  always  takes  the  place  of  two  atoms  of  hydro- 
gen, and  half  an  atom  of  calcium  is  impossible,  the 
formula  Ca3(PO4)2  is  the  only  possible  expression 
for  the  neutral  calcium  phosphate.  This  is  seen 
more  clearly  if  we  expand  the  formulae  (compare 
p.  183)  so  as  to  show  the  three  hydroxyl  groups 
and  the  replaceable  hydrogen  atoms  of  the  phosphoric 
acid. 


MAKING   SUPERPHOSPHATE.  2OQ 

^OH  ^0-Na  Pod)>Ca 

PO— OH  PO— O— Na  ^!>Ca 

^OH  ^-O— Na 


Tribasic  Sodium  salt  of  tribasic  Calcium  salt  of  tribasic 

phosphoric  acid.  phosphoric  acid.  phosphoric  acid. 

These  formulae  make  clear  to  us  the  replacement  of 
the  three  hydrogen  atoms  of  the  three  hydroxyl  groups 
of  phosphoric  acid  by  three  atoms  of  sodium,  and  the 
replacement  of  six  atoms  of  hydrogen  by  three  atoms 
of  calcium.  The  calcium  salt  of  tribasic  phosphoric 
acid,  which  is  found  native,  is  a  neutral  salt,  and  is 
insoluble  in  water  ;  but  if  we  remove  two  of  the  three 
atoms  of  calcium,  by  acting  on  the  salt  with  sulphuric 
acid,  an  acid  calcium  phosphate  is  produced  :  for  a 
salt  which  contains  replaceable  atoms  of  hydrogen  in 
addition  to  a  metal  —  in  this  case  one  atom  of  calcium  — 
is  called  an  acid  salt  (see  p.  158);  and  in  this  case  the 
sulphuric  acid  which  is  made  to  react  with  the  neutral 
calcium  phosphate  brings  about  the  replacement  of 
calcium  by  hydrogen.  The  sulphuric  acid  exchanges 
hydrogen  for  calcium,  and  forms  calcium  sulphate,  a 
salt  that  is  commonly  known  as  gypsum.  The  following 
equation  shows  this  exchange  of  atoms  :  — 


H2SO.  -        +  CaSO. 

One  mol.           two  mols.  one  mol.          two  mols. 

neutral  calcium  +  sulphuric  acid  calcium  +   calcium 

phosphate               acid  phosphate         sulphate. 


210      INTRODUCTION    TO   MODERN    CHEMISTRY. 

The  acid  calcium  phosphate  which  is  formed  by  the 
reaction  of  two  molecules  of  sulphuric  acid  with  one 
molecule  of  the  salt  present  in  bones,  or  phosphorite,  is 
soluble  in  water ;  hence  it  can  be  absorbed  by  the  plant 
roots,  and  is  capable  of  acting  as  an  artificial  manure. 
If  three  molecules  of  sulphuric  acid  were  allowed  to 
react  with  one  molecule  of  the  salt  in  the  raw  material, 
the  whole  of  the  calcium  would  be  removed  by  the 
sulphuric  acid,  and  free  phosphoric  acid  would,  of 
course,  be  obtained.  But  phosphoric  acid  is  so 
corrosive  that  it  would  destroy  the  roots  of  the  plants. 
What  is  called  manuring  with  phosphoric  acid  is  really 
manuring  with  acid  monocalcium  phosphate,  which  is 
the  substance  in  "  superphosphate "  that  acts  as  a 
fertiliser.  The  phrase  in  common  use,  "manuring  with 
phosphoric  acid,"  is,  therefore,  incorrect. 

It  is  easy  to  calculate  the  quantity  of  sulphuric  acid 
which  must  be  added  to  a  specified  quantity  of  phos- 
phorite in  order  to  produce  superphosphate.  The 
formula  of  neutral  tricalcium  phosphate  is  Ca3P2O8 ; 
and  as  P  =  31,  Ca  =  40,  and  O  =  16,  the  molecular 
weight  of  this  salt  is — 

Ca3  P2  08 

(3  x  40)  +  (2  x  31)  +  (8  x  16)  =  310. 

As  two  molecules  of  sulphuric  acid  must  be  used, 
and  as  the  molecular  weight  of  sulphuric  acid  (H2SO4) 
is  98,  it  follows  that  196  parts  by  weight  of  sulphuric 
acid  must  be  poured  on  to  310  parts  by  weight  of 
phosphorite  [assuming  the  phosphorite  to  be  pure 
tricalcium  phosphate]  :  stated  in  percentages,  63*2  kilos. 


MAKING   SUPERPHOSPHATE.  211 

sulphuric  acid  must  be  added  to  100  kilos,  phosphorite* 
[or  63*2  Ib.  or  cwts.  sulphuric  acid  to  100  Ib.  or  cwts. 
phosphorite]. 

In  manufacturing  superphosphate,  finely  ground 
phosphorite  is  thoroughly  mixed  with  the  calculated 
quantity  of  sulphuric  acid  ;  the  mass  is  heated  by  the 
heat  produced  in  the  reaction,  and  after  cooling  [and 
breaking  into  pieces,  and  powdering]  it  is  obtained  in 
the  form  of  a  powder  which  may  be  spread  on  the 
fields.  The  gypsum  that  is  produced  by  decomposing 
the  crude  phosphate  remains  mixed  with  the  soluble 
acid  phosphate :  but  that  does  not  matter,  as  the 
gypsum  does  not  injure  the  plants ;  indeed,  the  pre- 
sence of  calcium  salts  may  be  useful  in  the  cases  of 
soils  that  are  poor  in  lime,  for  we  know,  from  the 
analyses  of  plant  ashes,  that  compounds  of  calcium  are 
required  by  plants.  "  Superphosphate,"  then,  is  a 
mixture  of  monocalcium  phosphate  and  calcium  sulphate. 

The  potash  salts  that  are  needed  for  artificial 
manuring  may  be  obtained  from  the  Stassfurt  salts 
(compare  p.  58).  The  main  potassium  compound  in 
these  salts  is  the  double  salt  of  potassium  chloride 
and  magnesium  chloride,  which  has  the  composition 
KC1  .  MgCl2 .  6H2O.  Before  it  can  be  sold  to  the 
farmers  the  potassium  chloride  must  be  separated  from 
the  magnesium  chloride,  as  the  latter  is  harmful  to  plant 
growth.  This  is  effected  by  re-crystallising  the  carnal- 
lite,  which  is  the  mineralogical  name  of  the  double  salt. 

*  Calculated  by  the  proportion,  310  :  196  =  100  :  x,  whence 
x  =  63-2. 


212      INTRODUCTION    TO   MODERN   CHEMISTRY. 

Potassium  chloride  and  magnesium  chloride  both  dis- 
solve in  water,  but  potassium  chloride  is  much  less 
soluble  in  cold  water  than  magnesium  chloride.  In 
order  to  obtain  potassium  chloride,  therefore,  the  crude 
salt  from  the  mines  is  dissolved  in  hot  water,  and 
potassium  chloride  crystallises  out  as  the  solution  cools, 
while  magnesium  chloride  remains  dissolved  in  the 
mother-liquor.  As  there  is  very  little  demand  for 
magnesium  chloride,  this  mother-liquor  must  be  allowed 
to  run  into  the  nearest  stream.  A  relatively  small 
quantity  of  the  mother-liquor  suffices  to  yield  all  the 
bromine  that  is  required  on  the  market  (compare  p.  59) ; 
what  remains  after  the  bromine  has  been  extracted  is 
thrown  away.  A  double  sulphate  of  potassium  and 
magnesium,  called  kainite,  and  having  the  composition 
K2SO4  .  MgSO4  .  6H2O,  is  one  of  the  salts  found  at 
Stassfurt.  Potassium  sulphate  is  obtained  from  this 
salt. 

Now  that  we  have  considered  the  constituents  of 
plant-ashes  sufficiently  for  our  purpose,  we  shall  turn 
to  the  other  substances  that  are  required  for  the  growth 
of  plants.  The  most  important  of  these  is  the  element 
carbon,  which,  indeed,  is  the  very  foundation  of  the 
existence  of  plants.  This  carbon  makes  its  appearance 
when  parts  of  plants  are  heated  out  of  contact  with  air, 
that  is,  under  conditions  such  that  complete  combustion 
is  impossible  because  of  lack  of  oxygen.  If  wood  is 
heated  out  of  contact  with  air,  carbon  remains  in  the 
form  of  charcoal,  and  we  have  a  visible  demonstration 
of  the  fact  that  carbon  forms  a  large  part  of  the 
substance  of  a  plant.  It  is  clear  why  carbon  cannot 


NUTRITION    OF   PLANTS.  213 

be  present  in  plant  ashes>  and  is  not  therefore  found 
among  those  constituents  of  plant  ashes  which  are 
discovered  by  analyses  thereof:  for,  as  the  ashes 
are  obtained  by  burning  the  plant  in  a  crucible  in 
contact  with  the  air,  the  carbon  is  burnt  to  carbon 
dioxide,  which  passes  away  into  the  atmosphere.  As 
with  the  carbon,  so  with  the  nitrogen  and  the  water 
that  are  contained  in  plants ;  these  also  pass  away 
during  the  process  of  burning,  and  therefore  they  are 
not  found  among  the  constituents  of  the  ashes. 

Plants  obtain  their  carbon  from  the  atmosphere 
Although  the  air  contains  only  three  parts  of  carbon 
dioxide  gas  per  ten  thousand,  nevertheless  this  quantity, 
which  seems  so  small,  is  amply  sufficient  for  the  needs 
of  plants.  The  plants  absorb  the  carbonic  acid  gas  by 
their  leaves,  and  the  surface  presented  by  these  is  very 
large  compared  with  that  of  the  whole  of  the  other 
parts  of  a  plant.  It  is  the  green  colouring  material 
of  the  leaves,  called  chlorophyll,  which  is  able  to  make 
the  carbonic  acid  gas  of  the  atmosphere  available  for 
the  plants.  That  gas  seems  to  be  changed  directly 
into  starch,  a  substance  which  is  present  in  all  green 
leaves.  It  is,  of  course,  impossible  to  manure  fields 
with  carbonic  acid  gas,  to  supply  the  soil  artificially 
with  this  gas ;  fortunately  it  is  unnecessary,  for  no  one 
has  noticed  at  any  time  that  plants  suffer  from  want 
of  carbon  dioxide. 

Plants  take  the  hydrogen  they  require  from  the 
moisture  in  the  soil,  and  the  supply  of  moisture  is 
replenished  by  the  rain.  They  receive  sufficient 


214      INTRODUCTION    TO   MODERN   CHEMISTRY. 

oxygen  in  the  carbonic  acid  gas  (CO2)  and  the  water 
(H2O)  which  they  absorb ;  indeed,  they  get  more 
oxygen  than  they  need,  and  so  they  discard  some  of 
it  through  their  leaves.  The  oxygen  which  is  expired 
in  that  way  is  a  portion  of  that  taken  from  the  carbon 
dioxide,  not  that  taken  from  the  water :  thus  it  is 
that  plants  breathe  in  carbon  dioxide  (CO2)  and  breathe 
out  oxygen. 

As  we  know,  the  reverse  of  these  processes  occurs  in  the 
animal  kingdom.  Animals  inspire  air,  and  hence  oxygen,  by 
their  lungs  or  gills,  and  expire  carbon  dioxide.  It  is  the  oxygen 
which  is  breathed  in  with  the  air  that  forms  this  carbon  dioxide. 
The  oxygen  acts  on  compounds  in  the  animal  bodies  which 
contain  carbon,  and,  combining  with  their  carbon,  produces 
carbonic  acid  gas. 

The  circumstances  connected  with  the  absorption  of 
nitrogen  by  plants  are  very  remarkable.  Ten  thousand 
parts  of  air  contain  only  three  parts  of  carbon  dioxide, 
but  nearly  eight  thousand  parts  of  nitrogen  (see  p.  1 20). 
Although  that  small  quantity  of  carbon  dioxide  is  quite 
sufficient  to  supply  the  demands  of  plants,  yet  plants 
often  lack  nitrogen,  and  their  growth  is  stopped  for 
want  of  that  element,  notwithstanding  the  vast  quan- 
tities of  it  that  are  present  in  the  air.  The  reason 
for  this  must  be  that  the  absorption  of  the  nitrogen  of 
the  air  is  attended  with  difficulties. 

Nitrogen  is  required  by  plants,  because  it  is  ab- 
solutely necessary  for  the  production  of  albuminous 
substances.  Egg-white  is  the  type  of  a  large  class  of 
extraordinarily  complex  nitrogen-containing  substances, 
called  albuminoids,  which  are  produced  in  living  organ- 
isms, and  are  necessary  for  the  continuance  of  life  both 


NUTRITION   OF   PLANTS.  215 

in  the  animal  and  the  vegetable  world.  It  is  impossible 
to  think  of  life  without  the  presence  of  such  nitrogenous 
substances. 

Plants  are  not  furnished  with  an  organ  adapted  for 
taking  up  the  nitrogen  of  the  air  directly  quite  in 
the  same  way  as  they  absorb  carbon  dioxide.  On 
the  contrary,  they  only  absorb  nitrogenous  substances 
by  their  roots  from  the  soil.  Such  substances 
which  are  to  act  as  nourishment  to  plants  must  be 
soluble  in  water,  just  as  phosphates  must  be  soluble  in 
water.  Nitrogen  itself  is  practically  insoluble  in  water. 
For  a  long  time  we  did  not  know  the  processes 
whereby  nitrogen,  which  is  so  little  disposed  to  com- 
bine with  other  elements,  is  changed  into  compounds 
that  are  soluble  in  water,  and  therefore  can  be  ab- 
sorbed by  the  roots  of  plants.  On  the  other  hand, 
experience  showed  that  the  fertility  of  a  soil  was 
increased  by  the  addition  of  nitrogenous  bodies,  a  fact 
not  to  be  wondered  at  considering  what  we  know 
of  the  albuminoids  of  plants.  There  are  two  main 
materials  suitable  for  this  purpose,  and  with  both  of 
these  we  are  already  acquainted.  One  is  Chili  salt- 
petre— that  is  sodium  nitrate,  NaNO3  (see  p.  172) — a 
compound  which  is  very  soluble  in  water,  and  there- 
fore very  suitable  for  absorption  by  the  roots  of  plants, 
and  is  also  sufficiently  cheap  for  manuring  purposes  ; 
the  other  is  ammonium  sulphate,  (NH4)2SO4,  which  is 
obtained  from  the  gas-works  (see  forward  p.  271). 

The  method  whereby  the  nitrogen  of  the  air  is 
converted,  in  nature,  into  compounds  that  are  soluble 


2l6      INTRODUCTION    TO    MODERN    CHEMISTRY. 

in  water  has  been  elucidated  only  in  the  course  of  the 
last  fifteen  years.  The  help  of  bacilli  is  required  for 
effecting  this  change.  Arable  soils  contain  bacilli 
which  are  able,  by  their  life-processes,  to  combine 
nitrogen  with  oxygen  and  to  produce  compounds 
allied  to  nitric  acid.  These  compounds,  as  one  would 
suppose,  immediately  react  with  the  lime  or  the  potash 
compounds  in  the  soil  to  form  calcium  nitrate  and 
potassium  nitrate  [or  compounds  allied  to  these], 
which  are  soluble  in  water  :  in  this  way  the  nitrogen 
which  was  before  in  the  atmosphere  becomes  capable 
of  absorption  by  the  plant-roots.  But  such  bacilli  do 
not  collect  on  the  roots  of  all  kinds  of  plants  in 
quantities  sufficient  to  render  unnecessary  the  use  of 
nitrogenous  manures;  hence  the  increase  in  growth  which 
results  from  adding  soluble  nitrogenous  compounds, 
in  the  form  of  artificial  manure,  to  the  supply  of  such 
compounds  which  nature  provides  by  the  processes 
we  have  described.  It  is  for  this  reason  that  Chili 
saltpetre,  and  the  like,  are  found  to  be  so  useful  in 
increasing  the  fertility  of  soils.  The  bacilli  which  are 
found  on  the  roots  of  the  leguminous  plants  are  the 
best  nitrogen-collectors. 

Certain  nitrogenous  compounds  which  are  insoluble 
in  water  are  good  manures ;  such  are  dried  and 
powdered  blood,  and  shavings  of  horn,  both  of  which 
must  be  classed  with  the  albuminoids,  for  they  consist 
of  very  complex  nitrogenous  compounds  produced  by 
changes  in  living  organisms.  These  manures  act  more 
slowly  than  Chili  saltpetre  or  ammonium  sulphate, 
which  are  soluble  in  water,  because  they  have  to  be 

I 


PROPERTIES   OF   ARSENIC.  217 

changed  into  compounds  soluble  in  water,  and  this  is 
done  gradually  as  the  substances  decay  in  the  soil. 

We  have  dwelt  at  some  length  on  the  connections 
between  non-living  materials  and  the  plant-world, 
which  is  the  foster-mother  of  the  animal  world.  It 
is  easy  to  see  that  it  was  only  by  such  purely  chemical 
knowledge  that  agriculture  could  cease  to  be  an 
empirical  and  mechanical  trade  and  become  a  skilled 
art.  The  chemical  investigations  that  have  led  to 
this  knowledge  are  not  yet  fifty  years  old  ;  and  that 
is  the  reason  why  the  thorough  application  of  that 
knowledge  has  not  long  ago  spread  over  the  whole 
world,  but  is  still  confined  to  a  few  highly  civilised 
countries,  and  even  there  to  that  part  of  the  agri- 
cultural population  which  is  really  intelligent. 


ARSENIC. 

Arsenic  is  related  to  phosphorus  and  nitrogen.  Not- 
withstanding its  metal-like  appearance,  it  belongs  to 
the  same  group  as  these  two  non-metallic  elements ;  in 
the  first  place  because  it  combines  with  hydrogen  to 
form  a  gaseous  compound — arsenuretted  hydrogen — 
whereas  metals  do  not  form  such  hydrogen  compounds, 
and  secondly  because  its  compounds  with  oxygen 
yield  acids,  whereas  most  of  the  oxides  of  the  true 
metals  are  bases. 

It  is  only  those  metals  which  combine  with  very  large  quan- 
tities of  oxygen  that  form  both  basic  oxides  arid  also  compounds 
very  rich  in  oxygen  which  show  acidic  properties.  For  instance. 


2l8      INTRODUCTION    TO    MODERN    CHEMISTRY. 

manganese  forms  not  only  manganous  oxide,  manganic  oxide, 
and  manganese  peroxide,  but  also  both  manganic  and  permanganic 
acids  (see  forward,  under  MANGANESE). 

Arsenic  itself  is  found  native.  Compounds  of  arsenic 
with  sulphur,  with  oxygen,  and  with  metals  also  occur 
in  rocks;  and  these,  as  well  as  native  arsenic,  are 
worked  in  mines. 

Arsenuretted  hydrogen,  AsH3,  can  be  formed  by  a 
reaction  similar  to  that  which  yields  sulphuretted 
hydrogen.  As  iron  sulphide  reacts  with  hydrochloric 
acid  to  give  sulphuretted  hydrogen  gas  (see  p.  144), 
so  zinc  arsenide  (which  is  more  easily  made  than 
iron  arsenide)  reacts  with  hydrochloric  acid  to  give 
arsenuretted  hydrogen.  The  equation  which  expresses 
the  latter  reaction  is  this  :  — 


Zn3As2       +        6HC1        =        2AsH:s        + 

~.  .,          hydrochloric         arsenuretted  .         ,  ,     •  , 

Zinc  arsenide  4-      ?    add  -  hydrogen  gas  +    zinc  chloride. 

Many  other  compounds  of  arsenic  yield  arsenuretted 
hydrogen  gas  when  they  are  brought  into  a  mixture 
which  is  producing  hydrogen  —  that  is,  when  they  come 
into  contact  with  nascent  hydrogen  (see  p.  106).  It  is 
on  this  reaction  that  the  process  for  detecting  arsenic 
in  cases  of  poisoning  is  based.  That  process,  which 
is  more  certain  and  easier  of  performance  than  the 
detection  of  almost  any  other  poison,  is  conducted  as 
follows.  Pure  hydrogen  is  produced  in  a  flask  (A, 
fig.  50),  and  the  liquid  to  be  tested  for  arsenic  is  poured 
into  the  flask,  by  the  funnel  B.  The  gas  which  issues 
from  the  flask  is  ignited  at  c,  and  a  porcelain  plate  is 
held  in  the  flame.  If  arsenic  is  present  it  is  deposited 


DETECTION    OF  ARSENIC. 


219 


on  the  plate  in  black  spots.  The  tube  through  which 
the  gas  passes  has  been  narrowed  at  D  ;  that  tube  is 
now  heated  by  a  Bunsen  burner  placed  a  little  behind 
the  narrow  part.  Arsenuretted  hydrogen  is  decom- 
posed, at  a  moderately  high  temperature,  into  its 
constituents,  arsenic  and  hydrogen.  The  hydrogen 
escapes,  and  the  arsenic  appears  on  the  cooler  part  of 
the  tube,  in  front  of  the  flame,  as  a  lustrous  deposit 
known  as  "an  cfrsenic  mirror"  As  this  mirror  is 


Fig  50.— Detection  of  arsenic. 


obtained  even  when  a  mere  trace  of  arsenic  is  placed 
in  the  vessel  wherein  hydrogen  is  generated,  the 
detection  of  arsenic  is  easy  and  certain. 


Arsenic  combines  with  chlorine,  bromine,  and  iodine 
Two  compounds  of  arsenic  with  oxygen  are  known, 
corresponding  with  the  two  oxides  of  phosphorus, 
namely,  As2O3,  called  arsenic  trioxide,  and  As2O5,  called 
arsenic  pentoxide.  As  these  bodies  combine  with  bases, 
in  the  presence  of  water,  to  form  salts,  they  are  also 


220      INTRODUCTION    TO   MODERN   CHEMISTRY. 

called  arsenious  anhydride  and  arsenic  anhydride  re- 
spectively. 

Arsenious  anhydride,  As2O3,  is  popularly  known  as 
"  white  arsenic " ;  it  is  often  used  as  a  poison  for  rats. 
It  is  produced  in  quantity  as  a  bye-product  in  the 
working  of  arsenical  ores,  which  contain  such  metals 
as  cobalt,  nickel,  tin,  and  silver  in  addition  to  arsenic. 
The  ores  are  heated  in  furnaces  in  a  stream  of  air ;  the 
arsenic  is  burnt  to  arsenic  trioxide,  which  is  volatile  at 
the  high  temperature  of  the  furnaces  : — 

2As        +        30         =  As2O3. 

Arsenic        +        oxygen       =     arsenic  trioxide. 
(from  the  air) 

The  vapours  that  come  from  the  furnace  are  passed 
through  long  chambers  before  reaching  the  chimney, 
and  the  arsenic  trioxide  settles  on  the  bottoms  and 
sides  of  these  chambers.  In  this  way  the  poisonous 
substance  is  prevented  from  passing  out  by  the 
chimney.  The  price  of  arsenious  oxide  is  very  low, 
as  the  substance  is  almost  a  waste  product  in  certain 
metallurgical  operations. 


ANTIMONY. 

The  symbol  for  antimony,  Sb,  is  derived  from  the 
Latin  name,  stibium.  Antimony  is  more  metallic  than 
arsenic.  Its  applications  are  those  of  a  metal ;  it  is 
employed  chiefly  for  melting  with  lead,  to  which  it 
imparts  a  greater  hardness.  In  its  chemical  relations 
antimony  is  classed  with  the  non-metals,  inasmuch 
as,  like  nitrogen,  phosphorus,  and  arsenic,  it  forms  a 


PROPERTIES   OF   ANTIMONY.  221 

gaseous  compound  with  hydrogen — antimonuretted 
hydrogen,  SbH3.  The  behaviour  of  this  gas  is  very 
similar  to  that  of  arsenuretted  hydrogen.  Like  phos- 
phorus and  arsenic,  antimony  forms  two  compounds 
with  oxygen — antimony  trioxide,  Sb2O3,  and  antimony 
pentoxide,  Sb2O5.  The  composition  of  the  trioxide, 
Sb2O3,  corresponds  with  that  of  phosphorus  trioxide  or 
phosphorous  anhydride,  P2O3 :  but  antimony  trioxide 
does  not  show  acidic  properties,  although  it  is  the  oxide 
of  a  non-metal ;  it  reacts  with  acids  to  form  salts,  like 
the  oxides  of  metals ;  for  instance,  with  sulphuric  acid 
it  forms  antimony  sulphate  ;  hence  it  is  classed  with  the 
bases.  The  higher  oxide  of  antimony — that  is,  the 
oxide  richer  in  oxygen — Sb2O5,  which  corresponds  with 
phosphorus  pentoxide  or  phosphoric  anhydride,  P2O6, 
is,  however,  an  acid  anhydride ;  it  forms  salts  by 
reacting  with  bases  in  the  presence  of  water ;  for 
instance,  with  caustic  potash  solution  it  forms  the  fairly 
insoluble  salt  potassium  antimonate. 

Nature,  as  is  her  custom,  has  drawn  no  hard  and 
fast  line  between  the  metals  and  the  non-metals.  Anti- 
mony belongs  to  both  classes ;  in  the  gaseous  compound 
antimonuretted  hydrogen  and  in  the  acidic  pentoxide 
it  behaves  like  a  non-metal,  and  in  the  basic  trioxide  it 
behaves  like  a  metal. 


WE  now  come  to  the  last  group  of  elements  we  shall 
consider  which  contains  non-metals.  Some  of  the 
members  of  this  group  are  distinctly  metallic.  The 
group  comprises  the  four  elements  carbon,  silicon, 
germanium,  and  tin. 


CARBON. 

Carbon,  the  symbol  for  which  is  C,  is  found  in  nature 
in  three  modifications.  One  of  these  has  no  definite 
form ;  it  is  amorphous :  this  modification  of  carbon  is 
found  in  wood  charcoal.  The  two  other  modifications 
are  crystalline,  and  occur  in  very  much  smaller  quan- 
tities. If  the  crystals  take  the  form  of  black  leaflets 
they  are  called  graphite,  if  they  are  clear  and  trans- 
parent they  are  called  diamond. 

The  most  convenient  method  of  proving  that  diamond 
and  graphite  are  pure  carbon  is  to  burn  them  in  pure 
oxygen  gas,  and  to  show  that  the  sole  product  in 
either  case  is  carbon  dioxide,  CO2 ;  hence  neither  can 
be  anything  but  carbon.  Both  these  forms  of  carbon 
are  exceedingly  difficult  to  burn  in  air :  we  may  recall 
the  use  of  graphite  crucibles  in  melting  metals. 

Let  us  perform  the  combustion  of  a  diamond,  in  the 


BURNING  DIAMOND   IN   OXYGEN. 


223 


manner  shown  in  fig.  5 1 .  The  flask  A  is  closed  by  a 
cork,  through  which  passes  a  glass  tube  that  reaches 
to  the  bottom  of  the  flask.  Oxygen  is  passed  through 
the  flask,  from  a  gasholder,  *G,  and  then  into  an  open 
vessel,  B  (by  the  glass  tube  D,  which  passes  only  a  little 
way  through  the  cork  into  the  flask  A).  Some  clear, 


Fig.  51. — Burning  diamond  in  oxygen,  and  detection  of  the  carbon  dioxide 
produced. 

filtered  lime  water  is  placed  in  the  flask  A.  As  long 
as  oxygen  only  is  passing  through  the  apparatus  the 
lime  water  is  not  changed,  although  the  oxygen 
bubbles  through  it ;  for  what  can  be  formed  by  the 
mutual  action  of  these  two  materials  ?  Two  stout 
wires  of  copper  pass  through  the  cork  of  the  flask,  and 
these  are  connected,  within  the  flask,  by  a  spiral  of  thin 


224-     INTRODUCTION   TO   MODERN    CHEMISTRY. 


. 


platinum  wire.  When,  at  a  later  stage  of  the  experi- 
ment, an  electric  current  is  passed  through  the  copper 
wire — such  a  current  as  is  obtained  from  a  couple  of 
Bunsen  elements*  (E,  fig.  51)  is  quite  sufficient — the 
platinum  spiral  becomes  red  hot.  We  use  platinum 
because  we  know  that  our  spiral  will  not  melt,  not- 
withstanding the  thinness  of  the  wire,  inasmuch  as 
platinum  fuses  only  at  the  very  highest  temperatures. 
A  small  fragment  of  diamond  is  fastened  in  the  platinum 
spiral,  as  shown  on  an  enlarged  scale  in  fig.  51,  the 
cork  is  placed  tightly  in  the  flask  A,  and  oxygen  gas 
is  passed  through  the  apparatus.  After  a  short  time 
all  the  air  will  have  been  driven  out  of  the  flask.  We 
then  allow  the  stream  of  oxygen  to  pass  more  slowly, 
and  we  close  the  electric  circuit.  The  platinum  spiral 
gets  red  hot  and  glows  ;  the  heat  is  conducted  to  the 
diamond,  and  we  see  this  burning  brilliantly  in  the 
atmosphere  of  oxygen  and  gradually  disappearing.  At 
the  same  time  we  notice  the  lime  water  becoming 
turbid  ;  we  see  the  bubbles  of  gas  producing  a  white 
solid,  which  gives  a  milky  appearance  to  the  liquid. 
This  white  solid  gradually  settles  to  the  bottom  of 
the  flask ;  when  it  is  examined  it  is  found  to  be  nothing 
but  calcium  carbonate.  As  we  already  know  (p.  134), 
calcium  carbonate  is  insoluble  in  water;  therefore 
it  is  formed  by  the  reaction  between  the  gas  in  the 
flask,  which  is  no  longer  oxygen  but  carbon  dioxide, 
and  the  calcium  hydroxide,  Ca(OH)2,  present  in  the 
lime  water. 

*  The  word  element  has  here  a  meaning  entirely  different  from 
that  given  to  it  in  other  parts  of  this  book.  In  this  case  the  word 
has  nothing  to  do  with  chemistry. 


BURNING   DIAMOND   IN    OXYGEN.  225 


CO  +         Ca  CaC03         +  H0. 


Caroon  dioxide  gas  +  calcium  hydroxide  =  calcium  carbonate  +  water. 

The  carbon  dioxide  gas,  CO2,  must  be  produced  from 
the  diamond,  for  the  oxygen  gas  did  not  cause  the 
lime  water  to  become  turbid  before  the  diamond  began 
to  burn.  Hence  the  diamond  must  have  been  con- 
verted into  carbon  dioxide  by  burning,  and  must, 
therefore,  have  consisted  of  carbon. 

The  reason  for  setting  fire  to  the  diamond  by  the  somewhat 
unusual  method  of  placing  it  in  a  platinum  spiral  and  making 
the  spiral  red  hot  by  an  electric  current  is,  of  course,  that  it  is 
necessary  to  avoid  the  use  of  a  flame,  or  anything  of  that  sort  ; 
for  a  flame  would  have  formed  combustion-products  which  would 
have  mixed  with  the  products  of  the  combustion  of  the  diamond, 
and  it  would  not  have  been  possible,  without  further  complica- 
tions, to  decide  what  part  of  the  products  of  combustion  came 
from  the  diamond  and  what  part  came  from  the  material  which 
was  burnt  to  produce  the  flame. 

If  a  small  piece  of  graphite  were  burnt  in  place  of 
the  diamond,  and  in  the  same  manner  as  we  have 
burnt  the  diamond,  we  should  notice  exactly  the  same 
phenomena  ;  hence  graphite  also  is  nothing  but  carbon. 

This  process  of  burning  carbon  is  expressed  by  the 
following  equation  :-— 

C        +        O,  CO2. 

Carbon      -t-      oxygen       =     carbon  dioxide. 

As  the  atomic  weight  of  carbon,  C,  is  12,  and  that  of  oxygen, 
O,  is  1  6,  it  is  evident  that  when  12  parts  by  weight  of  carbon  are 
burnt  to  carbon  dioxide  44  parts  by  weight  of  that  gas  are 
produced  [CO2  "=  12  +  (2  x  16)  =  44].  If  these  numbers  are 

IS 


226      INTRODUCTION    TO   MODERN    CHEMISTRY. 

calculated  to  percentages,  it  is  seen  that  100  kilos,  of  carbon 
yield  366  kilos,  of  carbon  dioxide.  The  266  kilos,  of  oxygen 
which  are  required  to  combine  with  the  100  kilos,  of  carbon  are 
mixed  in  the  air  with  four  times  as  much  nitrogen — in  round 
numbers,  with  1066  kilos,  of  nitrogen.  Hence  366+1066  =  1432 
kilos,  [about  I2  tons]  of  gaseous  products  escape  by  a  chimney 
into  the  air  when  100  kilos,  [about  1 1  cwts.]  of  carbon  are  burnt 
in  a  fire,  assuming  that  no  more  air  is  drawn  through  the  fire 
than  suffices  to  burn  all  the  carbon.  The  work  done  by  the 
chimney  of  a  furnace  in  which  many  thousand  kilos,  of  carbon 
may  be  burnt  daily  is  evidently  considerably  greater  than  we  are 
accustomed  to  suppose. 

If  there  were  a  suitable  solvent  for  carbon,  there 
would  be  no  reason  why  the  carbon  should  not 
crystallise  therefrom  in  the  form  of  diamond.  But 
no  such  solvent  is  known.  Melted  iron  is  the  best 
solvent  of  carbon  ;  but  the  carbon  crystallises  as 
graphite  as  the  iron  cools.  Nevertheless,  it  has  been 
found  possible,  of  late  years,  to  cause  carbon  to 
separate  from  this  somewhat  extraordinary  solvent  in 
the  form  of  diamonds — of  almost  microscopic  small- 
ness,  it  is  true.  If  the  melted  iron  is  allowed  to  cool 
under  great  pressure,  instead  of  by  free  exposure  to 
the  air,  minute  diamonds  are  found  in  the  cold  block, 
along  with  graphite.  The  method  is  simple  enough. 
The  melted  iron  is  thrown  into  water  while  it  is  red 
hot ;  the  surface  solidifies  at  once,  and  as  it  contracts 
on  cooling  it  exerts  an  enormous  pressure  on  the  inner 
parts,  which  are  still  molten,  and  small  diamonds  are 
thus  produced.  In  order  to  get  the  graphite  and  the 
diamonds  out  of  the  iron  the  piece  of  iron  is  dissolved 
in  hydrochloric  acid ;  the  crystallised  carbon  remains, 
and  the  artificial  diamonds  are  separated  from  the 
graphite  by  picking. 


VALENCIES   OF   ELEMENTSr—  22/ 


ORGANIC  CHEMISTRY. 

The  capacity  of  carbon  to  form  compounds  is  greater 
than  the  combining  capacities  of  all  the  other  elements 
taken  together  :  the  compounds  of  this  one  element 
are  more  numerous  than  those  of  all  the  others. 
Nature  has  also  used  this  element  in  forming  living 
beings,  the  most  complicated  of  all  things.  Carbon  is 
found  in  all  materials  that  are  connected  with  the  world 
of  living  things.  The  analyses  made  by  chemists  had 
shown  this  long  ago  ;  hence  the  chemistry  of  carbon 
was  designated  "organic  chemistry"  and  this  special 
name  has  remained  in  use  till  to-day.  It  is  the  custom 
in  the  universities  and  colleges  to  begin  the  study  of 
chemistry  by  attending  lectures  on  inorganic  chemistry, 
which  deal  with  all  the  elements  except  carbon,  and  to 
follow  these  by  lectures  on  organic  chemistry. 

After  what  has  been  said  it  is  not  surprising  to  be 
told  that  very  great  efforts  have  been  required  to  set  in 
order  the  vast  number  of  compounds  of  carbon  which 
form  the  subject-matter  of  organic  chemistry.  Never- 
theless,. there  is  no  ver}'  great  difficult}'-  in  understanding 
the  fundamental  principles  by  the  application  whereof 
this  order  has  been  attained.  We  may,  indeed,  often  be 
inclined  to  think  that  the  formulae  of  the  organic  com- 
pounds, wherein  <(  rests  "  [or  radicles]  are  made  use  of 
(compare  p.  183),  recall  the  moves  of  the  pieces  on  a 
chess-board.  At  any  rate,  it  will  not  do  to  hurry 
through  the  following  thirty-five  pages. 

THE  VALENCIES  OF  THE  ELEMENTS. 
In  order  to  obtain  a  clear  understanding  of  this,  the 
most  pregnant  part  of  the  whole  domain  of  chemistry, 


228      INTRODUCTION    TO   MODERN    CHEMISTRY. 

we  must  turn  back  to  those  hydrogen  compounds  of 
the  non-metallic  elements  which  we  have  already  con- 
sidered so  fully.  The  hydrogen  compounds  of  the  four 
elements  chlorine,  bromine,  iodine,  and  fluorine,  which 
we  have  already  recognised  to  be  most  important  aids 
in  determining  atomic  weights,  are  these : — 

C1H          BrH  IH  FH. 

The  second  group  of  elements  wherewith  we  con- 
cerned ourselves  consisted  of  oxygen,  sulphur,  selenion, 
and  tellurium.  The  hydrogen  compounds  of  these 
elements  have  the  formulae  :— 

OH2        SH2        SeH2        TeH2. 

Nitrogen,  phosphorus,  arsenic,  and  antimony  formed 
our  third  group  of  elements.  The  hydrogen  compounds 
of  these  are  :  — 

NH3        PH3        AsH3        SbH3. 

If  we  place  the  formulae  of  these  compounds  together, 
writing  them  in  a  somewhat  different  fashion,  we  have 
the  following  presentation  : — 


First  Group. 
H-C1 

Second  Group. 
H—  0—  H 

Third  Group. 
H 

H—  N—  H 

H—  Br 

H—  S—  H 

H 
H—  P—  H 

H 

H— I  H— Se— H 

H— As— H 

H 
H-F  H-Te— H  | 

H— Sb— H 


VALENCIES   OF   ELEMENTS.  22Q 

The  mere  inspection  of  this  arrangement  shows  that 
the  elements  of  these  three  groups  have  something  in 
common.  This  common  property  is  that  [single  atoms 
of]  the  elements  of  the  first  group  bind  to  themselves 
one  atom  of  hydrogen,  [single  atoms  of]  the  elements 
of  the  second  group  hold  fast  two  atoms  of  hydrogen, 
and  [single  atoms  of]  the  elements  of  the  third  group 
unite  themselves  to  three  atoms  of  hydrogen.  The 
lines  joining  the  symbols  of  the  elements  and  the 
symbols  of  hydrogen  are  the  outward  expressions  of 
these  reactions.  Chemists  are  accustomed  to  use  the 
word  valency  to  designate  the  power  possessed  by  the 
atom  of  an  element  to  hold  to  itself  atoms  of  other 
elements,  and  to  measure  the  valency  [of  an  atom]  by 
the  number  of  atoms  of  hydrogen  which  it  can  bind 
to  itself.  The  [atoms  of  the]  elements  of  the  first 
group  are  said  to  be  monovalent,  those  of  the  second 
group  divalent,  and  those  of  the  third  group  trivalent. 

It  is  evident  from  what  has  been  said  that  the  atom 
of  hydrogen,  which  is  the  standard  of  valency,  is  itself 
monovalent.  For,  in  the  first  place,  the  attracting 
power  of  the  elements  is  reciprocal — one  atom  of 
hydrogen  is  able  to  hold  fast  one  atom  of  chlorine, 
just  as  one  atom  of  chlorine  holds -fast  one  atom  of 
hydrogen.  In  the  second  place,  the  atoms  of  the 
chlorine  group  are  themselves  monovalent,  because 
they  can  bind  each  but  one  atom  of  hydrogen ;  the 
atoms  of  the  oxygen  group  are  divalent,  because  they 
bind  each  two  atoms  of  hydrogen  ;  and  the  atoms  of 
the  nitrogen  group  are  trivalent,  inasmuch  as  they  can 
hold  fast  each  three  atoms  of  hydrogen. 


230      INTRODUCTION    TO   MODERN    CHEMISTRY. 

Chemists  had  got  about  as  far  as  this  in  their 
knowledge  of  these  relations  in  the  forties  of  the 
nineteenth  century.  Kekule  (who  died  in  1896)  put 
the  coping-stone  on  the  edifice  in  the  year  1857 
by  showing  that  organic  chemistry — the  chemistry 
of  the  innumerable  array  of  carbon  compounds — 
follows  the  same  law,  inasmuch  as  the  [atom  of] 
carbon  is  tetravalent.  With  the  insight  of  genius, 
Kekule  showed  that  all  the  carbon  compounds, 
vast  though  their  number  is,  can  be  derived  from 
the  hydrocarbon  CH4.  It  is  to  him  we  owe  the 
representation  of  this  hydrocarbon  by  the  formula 
which  puts  before  our  eyes  the  tetravalency  of  the 
carbon  atom  :  — 

H 

H— C— H 
H 

This  hydrocarbon  is  a  gas ;  it  is  called  marsh-gas, 
or,  more  generally  nowadays,  methane. 

As  thus  presented,  the  theory  of  the  valency  of  the 
elements  appears  so  simple  that  one  might  almost 
regard  it  as  self-evident.  It  is  difficult  to  comprehend 
the  endless  trouble  which  had  to  be  taken  by  genera- 
tions of  the  most  expert  chemists  before  this  clear 
insight  was  obtained  into  the  nature  of  the  mutual 
combinations  of  the  atoms. 

Although  we  have  said  but  little  as  yet  regarding  the  linking  of 
atoms,  we  are  in  a  position  to  give  an  opinion  concerning  the 


VALENCIES   OF    ELEMENTS.  231 

possible  existence  of  compounds,  a  thing  which  it  would  have 
been  impossible  to  do  with  our  previous  knowledge.  For  in- 
stance, we  can  assert  that  an  attempt  to  isolate  the  atomic 
complex  OH,  which  we  know  as  the  radicle  (or  rest)  of  water 
(p.  183),  would  be  fruitless.  For,  as  one  atom  of  oxygen  (O)  is 
able  to  hold  fast  to  itself  two  atoms  of  hydrogen  (2H),  the  com- 
pound OH  cannot  exist  by  itself.  Because  there  is  only  one 
atom  of  hydrogen  for  the  one  atom  of  oxygen  in  this  substance 
OH,  the  second  valency  of  the  oxygen  atom  (as  it  is  called)  has 
no  outlet ;  it  must  remain  suspended  in  the  air,  as  it  were  ;  and, 
as  thousands  of  investigations  have  shown,  this  is  a  thing  which 
never  occurs.  Just  as  little  could  the  rests  of  ammonia,  NH  or 
NH2,  exist  as  complete  compounds :  one  could  not  get  hold  of 
them.  For  in  these  combinations  only  one  atom  or  only  two 
atoms  of  hydrogen  are  present  for  one  atom  of  nitrogen  :  but 
the  nitrogen  atom  is  trivalent,  and  its  three  valencies  require 
three  atoms  of  hydrogen  for  their  saturation,  as  we  see  in 
ammonia,  NH3. 


The  construction  of  all  chemical  compounds,  whether 
inorganic  or  organic,  proceeds  in  accordance  with  the 
following  simple  rule,  wherein  is  found  the  key  by 
means  of  which  chemists  are  able  to  enter  and  survey 
the  immense  field  of  their  activity.  The  construction 
of  all  chemical  compounds,  from  the  simplest  (such  as 
hydrochloric  acid,  HC1)  to  the  most  complicated,  takes 
place  in  such  a  way  that  in  the  place  of  any  monovalent 
atom  or  group  there  enters  only  some  other  monovalent 
atom  or  group,  in  the  place  of  a  divalent  atom  or  group 
there  enters  only  some  other  divalent  atom  or  group,  in 
the  place  of  a  trivalent  atom  or  group  there  enters  only 
some  other  trivalent  atom  or  group.  The  monovalent, 
divalent,  and  trivalent  bodies  may  be  monavalent, 
divalent,  and  trivalent  atoms,  or  mono-,  di-,  or  tri- 
valent rests  or  atomic  complexes. 


232      INTRODUCTION    TO   MODERN    CHEMISTRY. 

The  readiest  method  for  making  clear  the  bearing 
of  this  simple  rule  on  the  linking  of  the  atoms  in  all 
known  compounds,  and  in  all  compounds  yet  to  be 
discovered,  is  to  give  some  examples.  Chlorine  is  a 
monovalent  element;  hence  it  is  able  to  replace 
hydrogen,  which  is  also  monovalent.  Consider  water, 
for  example,  H  —  O  —  H.  By  replacing  an  atom  of 
hydrogen  by  chlorine  we  obtain  the  compound 
Cl  —  O  —  H,  which  we  may  write  as  C1OH,  without 
the  joining  lines.  We  should  expect  this  compound 
to  exist,  and  we  should  expect  to  be  able  to  isolate  it. 
The  compound  is,  indeed,  well  known  ;  it  is  called 
hypochlorous  acid. 

The  calcium  salt  of  this  acid  —  calcium  hypochlorite  —  is  the 
active  substance  in  bleaching  powder  ;  hence  we  should  expect 
to  obtain  hypochlorous  acid  by  the  reaction  of  this  salt  with  a 
stronger  acid.  As  we  have  learned  (p.  48),  bleaching  powder 
is  formed  by  passing  chlorine  over  slaked  lime:  this  reaction 
produces  calcium  chloride  besides  calcium  hypochlorite.  The 
equation  which  expresses  this  reaction  also  indicates  that 
calcium  is  a  divalent  metal  ;  for  an  atom  of  calcium  holds  two 
atoms  of  chlorine  in  calcium  chloride,  and  two  atomic  groups 
of  hydroxyl  (OH)  in  slaked  lime  :  — 


2Ca<OH     +      2C1*        :    Ca<OCl      +   Ca< 


Cl     +    2H*°' 

Slaked  lime       +    chlorine  =        calcium       +    calcium       +    water. 
hypochlorite        chloride 

Bleaching  powder  is  a  compound  of  calcium  hypochlorite 
and  calcium  chloride. 

By  replacing  the  atoms  of  hydrogen  in  ammonia, 
NH3,  by  chlorine,  we  obtain  the  three  compounds 
NH2C1,  NHC12,  and  NC1S.  These  compounds  (called 


VALENCIES   OF   ELEMENTS.  233 

chlorides  of  nitrogen)  are  characterised  by  their  great 
readiness  to  explode. 

By  similar  operations  we  can  replace  three  atoms  of 
hydrogen  in  methane,  CH4,  a  compound  from  which 
all  the  compounds  studied  in  organic  chemistry  can 
be  derived.  One  of  these  derivatives  of  methane  is 
the  compound  CHC13,  which  is  called  trichloromethane. 
This  compound  was  known  long  before  Kekule  declared 
the  "constitution"  of  the  organic  compounds:  it  is 
generally  called  chloroform,  the  name  given  to  it  by 
Liebig,  who  discovered  it  in  the  year  1831.  Chloro- 
form is  much  used  in  medicine  as  a  sleep-producing 
agent.  It  can  be  obtained  directly  from  methane  by 
the  reaction  of  that  compound  with  chlorine:  — 


CH4       +        6C1        =        CHC13       + 
Methane      +     chlorine     =     chloroform      +      hydrochloric  acid. 

The  equation  shows  that  hydrochloric  acid  is  formed 
in  this  reaction,  besides  chloroform.  As  chloroform 
can  be  distilled,  it  is  easily  separated  from  the  other 
product  of  the  reaction  (compare  p.  201). 

Now  let  us  suppose  that  three  of  the  atoms  of 
hydrogen  in  methane  are  replaced  by  an  atom  of  tri- 
valent  nitrogen.  The  rule  that  has  been  stated  shows 
that  this  can  be  done.  The  product  of  this  reaction 
is  the  compound  H  —  C=~N  (or,  written  more  shortly, 
HCN).  This  compound  is  well  known  ;  it  is  the 
extremely  poisonous  prussic  acid.  We  have  here 
another  example  of  the  actual  existence  of  a  com- 
pound which  we  have  constructed  theoretically  on 
paper.  The  potassium  salt  of  this  acid,  KCN,  is 


234      INTRODUCTION    TO   MODERN    CHEMISTRY. 

called  potassium   cyanide*    and    hence    the  systematic 
name  of  the  acid,  HCN,  is  hydrocyanic  acid. 

Now  that  we  have  considered  the  replacement  of 
hydrogen  atoms,  singly  or  repeatedly,  by  the  atoms  of 
other  elements,  we  must  consider  their  replacement 
by  radicles  (or  rests).  This  will  give  us  an  opportunity 
of  showing  that  all  the  compounds  of  carbon — and  the 
number  of  them  is  enormous — follow  the  universal 
rule.  It  must  be  admitted  that  there  is  some  special 
property  belonging  to  carbon  which  makes  it  possible 
for  that  element  to  form  a  greater  number  of  compounds 
than  all  the  other  elements  together.  This  special 
property  is  the  following.  The  atoms  of  carbon  are  able 
to  combine  with  one  another  in  any  numbers,  whereas 
the  atoms  of  the  other  elements  are  either  unable  to 
do  this  or  do  it  only  to  a  very  limited  extent.  Atoms 
of  carbon  are  able  to  take  part  in  the  formation  of 
molecules  of  great  size — that  is  to  say,  molecules  that 
are  composed  of  a  very  large  number  of  atoms — 
whereas  molecules  that  are  formed  entirely  of  atoms 
of  other  elements  are  never  formed  of  very  many 
atoms.  Examples  will  best  show  us  how  molecules 
of  great  complexity  are  formed  with  the  aid  of  atoms 
of  carbon. 

It  is  evident  that  only  one  compound,  H — Cl,  can  be  formed 
by  the  union  of  monovalent  chlorine  with  monovalent  hydrogen  ; 
any  other  compound  is  impossible.  Two  compounds  of  divalent 
oxygen  with  hydrogen  are  known  :  water,  H2O  or  H — O — H, 
and  hydrogen  peroxide,  H2O2  or  H— O— O— H.  No  other 
compound  of  these  two  elements  has  been  formed.  Three  com- 

*  From  the  Greek  <vavos  =  dark  blue. 


VALENCIES   OF   ELEMENTS.  235 

pounds  of  hydrogen  with  trivalent  phosphorus  are  known  :  PH3  or 

/T-I 

P<   H,  P,H,  or  5>p— p<5,  and  P,H.,  or  H— P=P— P=P— H. 

\H 
Trivalent  nitrogen  also  forms  three  compounds  with  hydrogen : 

/  T_r 

ammonia,   NH3   or   N<^~H,    hydrazine,  N,H.  or  g>N— N<§' 
\H 

N\ 
and  hydrazoi'c  acid,  N3H  or    |   >N — H.     The  expanded  formulae 

N/ 

of  these  compounds  exhibit  the  arrangements  of  the  valencies 
between  the  atoms  of  hydrogen  and  those  of  oxygen,  phosphorus, 
and  nitrogen  respectively.  We  see  as  many  as  four  atoms  of 
phosphorus  arranged  in  a  chain,  some  of  them  held  by  double 
bonds  (compare  p.  239);  and  we  notice  the  same  kind  of  com- 
bination between  atoms  of  nitrogen  in  hydrazoi'c  acid.  No 
other  compounds  of  nitrogen  and  hydrogen  or  of  phosphorus 
and  hydrogen  have  been  obtained  save  the  three  in  each  case. 
We  find  at  most  four  atoms  of  phosphorus  or  three  atoms  of 
nitrogen  held  together  in  the  molecules  of  these  phosphorus  or 
nitrogen  compounds  :  all  attempts  that  have  been  made  to  bind 
together  more  than  these  numbers  have  been  fruitless.  On  the- 
other  hand,  we  shall  see  immediately  that  the  number  of  com- 
pounds formed  of  hydrogen  and  carbon  only — the  hydrocarbons 
— is  very  great. 


If  we  suppose  that  one  atom  of  hydrogen  is  taken 
away  from  the  molecule  of  methane,  CH4,  we  obtain 

H 

the  radicle  CH3,  or    „ p TT  ,    which   cannot    exist 

by  itself.  | 

This  radicle  requires  the  addition  of  an  atom  of 
hydrogen  to  convert  it  into  a  body  which  can  exist ; 
hence  it  can  bind  to  itself  an  atom  of  hydrogen.  And 
so  the  universal  rule  holds  good  here  also,  in  what  may 
be  called  an  inverted  sense :  that  which  can  hold  fast 


236      INTRODUCTION    TO   MODERN    CHEMISTRY. 

one  atom  of  hydrogen  is  monovalent.  Hence  CH3  is  a 
monovalent  radicle.  Now,  in  the  compounds  of  carbon 
almost  any  monovalent  atom  or  group  can  take  the 
place  of  another  monovaleut  atom  or  group  ;  hence,  for 
example,  the  radicle  CH3  is  able  to  take  the  place  of  H. 
This  radicle  is  monovalent,  hence  it  can  combine  with 
other  monovalent  radicles  or  atoms;  it  is  able  to 
combine  with  itself — to  bind  itself  to  itself,  so  to  speak. 
In  the  latter  case  two  monovalent  CH3  radicles  hold 
together  by  their  carbon  atoms ;  thus  : — 

H  H  H    H 

I  I  i       I 

H— C—    combines  with     — C— H      to  form    H— C— C— H. 

I  I  II 

H  H  H     H 

The  new  hydrocarbon,  C2H6,  which  is  thus  obtained  is 
called  ethane. 

Evidently  we  can  repeat  this  process,  as  the  number 
of  carbon  atoms  which  are  able  to  hold  together  in 
a  molecule  is  very  great,  in  contradistinction  to  the 
atoms  of  the  other  elements. 

H    H  H  H    H    H 

II  I  III 

H— C— C—  combines  with  — C— H     to  form  H— C— C— C— H. 

II  I  III 

H     H  H  H     H    H 

Monovalent  radicle  Monovalent  radicle  Propane, 

of  ethane.  of  methane. 

The  hydrocarbon,  C3H8,  which  is  thus  produced  is 
called  propane.  By  replacing  an  atom  of  hydrogen  in 
the  molecule  of  propane  by  the  radicle  CH3  we  obtain 
a  new  hydrocarbon,  called  butane. 


ISOMERIC   COMPOUNDS.  237 

H 


H— C— H 


H     H    H    H  H 

I       I       I   '   1  I 

H— C— C— C— C— H      and     H— C— C— H. 

H    H    H    H  H 

H— C— H 

Butane.  | 

H 

Isobutane. 

A  closer  examination  of  the  formula  of  butane  shows 
that  another  way  of  linking  the  atoms  in  this  molecule 
is  possible,  without  disturbing  the  tetravalency  of  the 
carbon  atoms  or  the  monovalency  of  the  hydrogen 
atoms.  There  are  thus  two  bodies  each  of  which  has 
the  empirical  formula  C4H10.  Such  compounds  are 
called  isomerides  ;  hence  the  designation  of  the  second 
butane  as  /sobutane.  Isomeric  compounds  are  composed 
of  the  same  numbers  of  the  same  atoms  (in  the  present 
case  of  four  carbon  atoms  and  ten  atoms  of  hydrogen)  ; 
nevertheless,  they  are  different  from  one  another,  inas- 
much as  the  arrangements  of  the  atoms  in  their  mole- 
cules are  not  the  same.  This  fact  makes  possible  the 
existence  of  a  vast  number  of  carbon  compounds 
which  did  not  come  into  our  consideration  before.  For 
instance,  eighteen  isomeric  octanes  are  possible,  all 
having  the  formula  C8H18  (see  forward,  p.  247).  Chains 
containing  as  many  as  sixty  atoms  of  carbon  have  been 
constructed  in  the  laboratory.  The  formula  of  the 
hydrocarbon  which  contains  this  number  of  atoms  of 
carbon  is  C^H^ 

Before  proceeding  farther  with  the  consideration  of 


238      INTRODUCTION   TO   MODERN   CHEMISTRY. 

the  formulae  wherein  the  atomic  complexes  seem  to  be 
moved  about  in  a  manner  that  recalls  the  movements 
of  the  pieces  in  chess  we  shall  pay  attention  to  certain 
actual  relations  between  compounds,  and  especially  to 
a  method  which  is  used  in  the  laboratory  for  forming 
propane,  starting  with  methane  and  ethane.  For  this 
purpose  chlorine  is  allowed  to  react  with  methane  and 
ethane,  which  hydrocarbons  are  directly  attacked  by 
that  element  :— 

CH4      +      C12      -         CH3C1       '  +  HC1. 

Methane    +    chlorine   =   methyl  chloride  +    hydrochloric  acid. 

C2H6     +       C12      =          C2H5C1       +  HC1. 

Ethane     -t-  chlorine   =     ethyl  chloride    -i-   hydrochloric  acid. 

As  the  equations  show,  the  products  of  these  re- 
actions are  methyl  chloride  and  ethyl  chloride  respec- 
tively. The  monovalent  radicles  (or  "  rests  ")  of  such 
hydrocarbons  as  methane,  ethane,  propane,  etc.,  are 
named  methyl,  ethyl,  propyl,  etc.  If  a  mixture  of  methyl 
chloride  and  ethyl  chloride  is  heated  with  sodium,  the 
sodium  withdraws  the  chlorine,  forming  sodium  chloride, 
and  the  two  radicles — methyl  and  ethyl — combine  to 
form  propane  : — 

H  H    H  H    H    H 

H— C— !  Cl  +  2Na  +  Cl  I— C— C— H    =  H— C— C— C— H  +  2NaCl. 

I      j II  III 

H  H    H  H    H    H 

Methvl  j.  ethyl  sodium 

*        +      sodium      +        ,   y. ,  =  propane  +     , 

chloride  chloride  chloride. 

In  this  reaction  we  recognise  the  synthesis  of  propane. 


The  foregoing  reactions  suggest  the  process  whereby  we  can 
pass    from  methane  to  ethane.       For  this  purpose  all  that  is 


CONSTITUTIONAL   FORMULA  239 

needed  is  to  heat  methyl  chloride  with  sodium.  Two  molecules 
of  methyl  chloride  react  with  sodium,  as  shown  in  the  following 
equation  : — 

2CH3C1        +     2Na     =   H3C— CH3    +          2NaCl. 
Methyl  chloride    +    sodium   =        ethane         -f    sodium  chloride. 

If  the  formulae  are  expanded,  we  have  : — 

H  H  H     H 

I  | :  I  I  I 

H— C—  j  Cl  +  2  Na  +  Cl  i-C— H     =     H— C— C— H  +  2NaCl. 
H  H  H     H 

Meth>'1      +      sodium      +     ""^y1       =  ethane          +    Sodium 

chloride  chloride  chloride. 

We  shall  shortly  become  acquainted,  with  the  method 
for  preparing  methane,  which  is  the  starting  point 
wherefrom  all  the  compounds  of  organic  chemistry  are 
theoretically  obtained. 

Let  us  suppose  that  two  atoms  of  hydrogen  are 
taken  away  from  methane;  we  obtain  the  "rest/'  or 
radicle,  CH2,  which  is  thus  written  in  an  expanded 

H 
formula  H — C — .     This  radicle  is,  of  course,  divalent, 

i 

for  it  is  able  to  combine  with  two  atoms  of  hydrogen. 
As  two  methyl  groups  are  able  to  combine  with  one 
another,  so  can  this  radicle  CH2  combine  with  itself. 
The  product  is  C2H4,  which  is  generally  written 
H2C~CH2,  the  divalency  of  the  CH2  group  being 
expressed  by  the  double  bond.  This  hydrocarbon, 
C2H4,  is  called  ethylene ;  in  the  same  series  we  have 
propylene,  H3C — CH=CH2,  etc.  It  is  evident  that  pro- 
pylene  may  be  represented  as  methyl-ethylene.  If  we 


240      INTRODUCTION    TO   MODERN    CHEMISTRY. 

think  of  methane  deprived  of  three  atoms  of  hydrogen , 
we  have  the  monovalent  radicle  HC_^.  If  one  atom 
of  the  trivalent  nitrogen  combines  with  this  radicle 
HCEE,  we  get  HCN,  which  is  prussic  acid,  a  compound 
whose  formula  has  been  already  deduced  in  a  some- 
what different  way  (compare  p.  233).  If  HCEE  com- 
bines with  itself,  which  must  be  possible,  we  obtain  a 
hydrocarbon  called  acetylene,  HCEECH  or  C2H2.  We 
shall  return  to  this  hydrocarbon,  which  has  been  so 
much  talked  of  recently,  when  we  are  dealing  with 
coal-gas;  meanwhile  we  have  learnt  its  formula  and 
"constitution." 

The  appearance  and  external  qualities  of  the  hydro- 
carbons are  determined  by  the  numbers  of  carbon 
atoms  in  the  molecules  of  these  compounds.  Methane 
is  a  gas,  so  are  ethane  and  propane  ;  but  hydrocarbons 
with  more  carbon  atoms  in  their  molecules  than  propane 
are  liquids — some  of  these  occur  in  petroleum ;  and  as 
the  number  of  carbon  atoms  increases,  the  hydrocarbons 
become  butter-like  and  then  solid.  Vaseline  consists 
of  hydrocarbons  of  the  consistency  of  butter,  paraffin- 
wax  of  solid  hydrocarbons. 

So  far  we  have  confined  our  attention  almost  com- 
pletely to  compounds  of  carbon  and  hydrogen,  that  is, 
to  hydrocarbons.  At  the  most  we  have  considered  com- 
pounds formed  by  the  replacement  of  hydrogen  atoms 
in  hydrocarbons  by  monovalent  chlorine,  Cl,  or  by 
trivalent  nitrogen,  N.  But  the  fundamental  rule  holds 
good  :  in  organic  chemistry  any  monovalent  atom  or 
group  can  replace  another  monovalent  atom  or  group, 
any  divalent  atom  or  group  can  replace  another  divalent 


CLASSES   OF   CARBON    COMPOUNDS.  241 

atom  or  group,  any  trivalent  atom  or  group  can  take 
the  place  of  another  trivalent  atom  or  group. 

Let  us  return  to  the  formula  of  water,  H — O — H. 
The  group  — O — H  (hydroxyl)  is  a  monovalent  radicle, 
inasmuch  as  it  is  able  to  bind  to  itself  a  single  atom  of 
hydrogen.  If  an  atom  of  hydrogen  in  methane  (CH4) 
is  replaced  by  hydroxyl,  the  compound  that  is  obtained 
is  CH3 — OH,  or,  written  more  fully, 

H 
H—  C-O-H. 


This  compound  is  an  alcohol.  All  carbon  compounds 
whose  molecules  contain  hydroxyl  groups  united  to 
carbon  in  the  way  shown  in  the  above  formula  are 
called  alcohols.  The  word  alcohol  is  used  by  the 
chemist  as  a  specific  name,  in  contradistinction  to  the 
practice  of  ordinary  life.  The  -special  alcohol  we  are 
dealing  with  at  this  moment  is  called  methylic  alcohol : 
evidently  it  is  the  simplest  alcohol  that  can  exist. 

Methylic  alcohol  has  been  known  for  many  years, 
as  it  is  obtained  by  heating  wood  in  closed  vessels — by 
the  dry  distillation  of  wood,  as  this  process  is  called 
(see  p.  269).  Because  of  its  preparation  from  wood,  this 
alcohol  was  called  wood  spirit.  At  the  time  of  its  dis- 
covery nothing  was  known  of  tetravalent  carbon,  or 
methyl,  or  ethyl.  The  names  wood  spirit  and  methylic 
alcohol  are  now  synonymous. 

If  one  hydrogen  atom  in  the  methyl  group  of  methylic 

16 


242      INTRODUCTION    TO   MODERN    CHEMISTRY. 

alcohol,  H3C — OH,  is  replaced  by  another  methyl  group 
(CH3),  we  obtain  ethylic  alcohol,  H3C— CH2  -OH.  But, 
in  accordance  with  our  rule,  it  is  evident  that  we  might 
replace  the  hydrogen  of  the  hydroxyl  group  in  methyl 
alcohol  (H3C — OH)  by  a  methyl  group.  The  result  of 
that  replacement  would  be  H3C— O — CH3.  This  com- 
pound, which  is  called  methylic  ether,  is  easily  prepared. 

We  are  constantly  being  presented  with  examples  of 
the  way  wherein  the  radicles,  or  "  rests,"  can  be  moved 
here  and  there.  Ethylic  alcohol  is,  so  to  speak,  the 
next  older  brother  of  methylic  alcohol.  Such  a  rela- 
tionship as  that  of  these  two  compounds  is  called  an 
homologous  relation  in  organic  chemistry.  Ethylic 
alcohol  is  the  next  higher  homologue  of  methylic 
alcohol ;  and,  as  we  can  now  easily  perceive,  propylic 
alcohol,  CH3— CH2— CH2— OH,  is  the  next  higher 
homologue  of  ethylic  alcohol.  The  process  may  be 
extended  immensely  ;  in  a  word,  there  is  a  vast  number 
of  alcohols  possible  to  the  chemist. 

H  H    H*  H    H    H 

H— C— OH        H— C— C— OH        H— C— C— C— OH 

&          iU  .U  A 

Methylic  alcohol.         Ethylic  alcohol.  Propylic  alcohol. 

Empirical  formula,  CH4O.  C2H6O.  C3H8O. 

Whisky  and  other  spirits  consist  [chiefly]  of  diluted 
ethylic  alcohol.  This  alcohol  is  present  in  all  fermented 
drinks,  and  gives  them  their  intoxicating  effects.  It  is 
contained  in  wine,  beer,  cider,  etc.  In  all  these  drinks 
it  is  produced  by  the  fermentation  of  fruit  sugar,  which 


CLASSES  OF   CARBON   COMPOUNDS.  243 

is  easily  obtained  from  other  sugars,  such  as  cane 
sugar,  and  is  produced  in  fermentable  liquids.  The 
[principal]  change  produced  in  fruit  sugar  during 
fermentation  is  expressed  by  the  equation : — 

C6H1206     =       2C2H60       +         2C02. 
Fruit  sugar  =  ethylic  alcohol  +  carbon  dioxide. 

Groups  or  classes  of  compounds,  such  as  the  alcohols, 
have  definite  class  properties.  For  instance,  all  these 
alcohols  behave  similarly  towards  oxygen :  when 
oxidised,  they  lose  two  atoms  of  hydrogen,  and  these 
combine  with  an  atom  of  oxygen  to  form  water,  H2O. 
A  single  atom  of  oxygen,  which  is  divalent,  then  takes 
the  place  of  the  two  atoms  of  hydrogen  that  leave 
the  molecule.  In  the  case  of  ethylic  alcohol  the  first 
part  of  this  change  is  presented  thus  : — 

H    iH  H 

I    1 1  I      /o 

H— C— C— O|H  +  O  |  K--C—C/      H-  H2O. 

H    H  H  H 

Ethylic  alcohol  +  oxygen      =      ethylic  aldehyde    +  water. 

We  have  again  symbolised  the  divalency  of  the  atom 
of  oxygen,  in  the  compound  produced  in  this  reaction, 
by  a  double  line.  We  have  placed  the  name  of  the 
compound  that  is  produced  under  the  formula  of  that 
compound.  The  compound  is  an  aldehyde  (abbreviated 
form  of  alcohol  dehydrogenatus).  This  special  compound 
is  ethylic  aldehyde.  Methylic  alcohol,  as  one  would 
suppose,  yields  methylic  aldehyde,  propylic  alcohol 
gives  propylic  aldehyde,  and  so  on. 


244      INTRODUCTION    TO   MODERN    CHEMISTRY. 

The   aldehydes  are  able  to   take  up  more  oxygen, 
whereby  the  hydrogen   atom    that   is    present   in   the 

/       ;^0\ 
aldehydic  group  I — C          I  is  changed  into  hydroxyl, 


/       ;^0\ 

KH)  " 


OH.     By  this  reaction  the  aldehydes  are  changed  into 
acids  ;  for  example  : — 

H  H 

I        ^O  1-0 

H— C— C          +  O     =      H— C— C 

I        -H  |        ^OH 

H  H 

Ethylic  aldehyde  -  oxygen    =         acetic  acid. 

The  acid  that  is  produced  in  this  case  is  the  well- 
known  acetic  acid,  the  empirical  formula  whereof  is 
C2H4O2.  The  formula  is  generally  written  H3C — COOH, 
because  this  formula  is  at  once  more  convenient  than 
the  fully  expanded  formula,  and  indicates  that  the  acid 
is  monobasic,  inasmuch  as  it  contains  only  one  hydroxyl 
group.  It  has  been  known  for  ages  that  acetic  acid  is 
formed  by  the  oxidation  of  alcohol.  Wine  or  beer 
becomes  sour  when  it  is  allowed  to  stand  in  the  air, 
wine-vinegar  or  beer-vinegar  being  produced.  The 
air,  or,  more  accurately,  the  oxygen  of  the  air,  converts 
the  alcohol  immediately  into  the  acid,  without  the 
formation  of  the  intermediate  substance  aldehyde ;  but 
aldehyde  is  readily  prepared  in  the  laboratory  by  the 
oxidation  of  alcohol. 

Two  salts  of  acetic  acid  demand  our  attention- 
acetate  of  lead,  and  acetate  of  lime,  or,  more  accu- 
rately, acetate  of  calcium.  As  lead  (Pb,  from  the 
Latin,  plumbum)  is  divalent,  the  formula  of  the  lead  salt 


CLASSES   OF   CARBON   COMPOUNDS.  245 

is  (H3C — COO)2Pb.  It  retains  from  olden  times  the 
name  sugar  of  lead,  because  it  has  a  sweet  taste  ;  but, 
despite  its  tempting  name,  like  all  other  lead  com- 
pounds, it  is  very  poisonous. 

Calcium  acetate,  (H3C — COO)2Ca,  is  the  material 
which  serves  for  the  preparation  of  acetone,  a  compound 
we  met  with  in  the  manufacture  of  smokeless  powder. 
If  calcium  acetate  is  subjected  to  dry  distillation — for 
instance,  if  it  is  heated  in  a  retort — it  decomposes  into 
calcium  carbonate  (CaCO3),  which  remains  in  the  retort, 
and  acetone,  C3H6O,  which  easily  distils  over,  as  it  is  a 
liquid  that  boils  at  58°  C.  [136-4°  F.].  This  decom- 
position of  calcium  acetate  is  more  easily  followed  if 
the  formulae  are  expanded,  thus  : — 

H3C— COiO I  H,Cv 

\Ca|       =  >C=0  -f          CaC03. 

H3C-|COO_     |  H3C 

Calcium  acetate  acetone       +  calcium  carbonate. 

In  this  sufficiently  expanded  formula  of  acetone  we 
again  recognise  the  tetra valency  of  the  atom  of  carbon. 
It  is  not  necessary  to  expand  the  CH3  groups ;  we 
already  know  that  CH3  is  a  monovalent  radicle,  and 
we  know  why  this  is  so. 

Ethylic  alcohol,  H3C— CH2— OH,  yields  acetic  acid, 
H3C— COOH.  Methylic  alcohol,  H— CH2— OH,  of 
course,  yields  th.e  corresponding  acid  H — COOH  ; 
this  compound  is  called  formic  acid,  because  it  is 
found  in  ants  (Latin,  formica  =  an  ant).  Propylic 
alcohol  gives  propionic  acid,  and  so  on.  We  see  that 
to  each  of  these  alcohols  there  belongs  an  aldehyde  and 
an  acid. 


246      INTRODUCTION   TO   MODERN    CHEMISTRY. 

The  replacement  of  hydrogen  by  radicles  of  various 
kinds,  in  the  manner  already  illustrated,  is  carried  on 
very  extensively  in  organic  chemistry. 

Ammonia  has  the  formula  NH3;  hence  NH2  is  a 
monovalent  radicle,  for  it  wants  one  atom  of  hydrogen, 
and  this  radicle  may  replace  one  atom  of  hydrogen. 
Let  us  take  the  simplest  case.  Let  this  radicle  NH2 
replace  an  atom  of  hydrogen  in  methane,  CH4 ;  then 
from  methane,  CH4,  we  obtain  methylamine,  CH3NH2. 

H  H 

H— C— H  H— C— NH, 

A  A 

Methane.  Methylamine. 

The  empirical  formula  of  methylamine  is  CNH5. 

Chains  of  carbon  atoms  need  not  be  arranged  in  one 
line ;  they  may  have  lateral  ramifications ;  for  hydro- 
gen atoms  in  a  single  chain  of  carbon  atoms  may  be 
replaced  by  methyl  groups,  etc.  The  following  hydro- 
carbon is  an  example  wherein  the  carbon  chains  divide 
at  two  places  : — 

H        H        H        H        H 

I          I          I          I          I 
H— C  —  C  —  C  —  C  —  C—  H 

I  !  I 

H  H  H 

H— C— H     H— C— H 

I  I 

H  H— C— H 

A 

The  empirical  formula  of  this  hydrocarbon  is  C8H18 ; 


FORMULA   OF   BENZENE.  247 

the  compound  is  an  octane.  The  single  chain  contains 
five  carbon  atoms ;  it  is,  therefore,  a  pentane  chain.  As 
one  hydrogen  atom  is  replaced  by  the  methyl  group 
CH3,  and  another  by  the  ethyl  group  C2H5,  the  hydro- 
carbon is  called  methyl  ethyl  pentane.  It  is  one  of  the 
eighteen  possible  octanes  (compare  p.  237). 

It  has  been  said  that  any  monovalent  atom  or  group 
is  replaceable  by  any  other  monovalent  atom  or  group, 
etc.  If  this  possibility  is  fully  taken  advantage  of,  a 
vast  number  of  compounds  may  be  formed,  all  of  which 
can  be  traced  back  to  chains  of  carbon  atoms  with 

lateral  ramifications. 

i 

From  what  has  been  said,  we  may  form  some  notion 
of  why  it  is  that  carbon  is  able  to  form  a  greater 
number  of  compounds  than  any  other  element;  an 
almost  indefinitely  large  number  of  atoms  of  this 
element  may  be  held  together  in  a  molecule.  But  not 
only  are  there  single  chains  of  carbon  atoms  and 
chains  with  side  branches  ;  there  are  also  ring-formed 
chains ;  and  the  number  of  compounds  derived  from 
these  is  greater  than  the  number  of  those  derived 
from  what  are  called  "open  chains."  There  are  very 
many  compounds  in  the  chemistry  of  carbon  from 
which  a  hydrocarbon  can  be  obtained  that  has  the 
formula  C6H6,  and  is  called  benzene,  because  benzoic 
acid  is  one  of  the  bodies  wherefrom  it  is  obtained. 
As  this  compound  contains  only  six  atoms  of  hydrogen, 
combined  with  six  atoms  of  carbon,  it  seems  scarcely 
possible  to  bring  it  into  accordance  with  the  tetravalency 
of  carbon.  This  was  done,  however,  by  Kekule  in 
1866,  who  thereby  placed  the  coping  stone  on  his 


248      INTRODUCTION   TO   MODERN    CHEMISTRY. 

building  of  organic  chemistry.  For  nothing  has  since 
been  found  in  the  domain  of  carbon  chemistry,  and  we 
may  assert  that  nothing  will  be  found,  which  does  not 
or  will  not  fit  into  this  wonderful  structure.  What 
remains  to  be  done  is  to  work  out  all  the  consequences 
that  follow  from  the  conceptions  of  Kekule.  Following 
Kekule,  let  us  regard  the  six  hydrogen  atoms  of 
benzene  as  equally  distributed  among  the  six  carbon 
atoms,  a  fact  which  has  been  established  by  experi- 
mental investigations  of  the  derivatives  of  benzene 
(with  some  of  which  we  shall  soon  become  acquainted), 
and  let  us  suppose  that  the  carbon  atoms  of  benzene 
are  alternately  singly  and  doubly  linked  to  one  another. 
We  arrive  at  the  following  single  chain  formula  :  — 

H    H    H    H    H    H 


The  four  middle  carbon  atoms  are  here  tetravalent  ; 
for  each  is  bound  to  its  two  adjacent  carbon  atoms 
by  three  valencies,  and  the  fourth  valency  of  each  is 
satisfied  by  hydrogen.  But  the  two  carbon  atoms 
at  the  ends  of  the  chain  seem  to  be  only  trivalent,  for 
they  are  bound  to  neighbouring  carbon  atoms  by 
double  linkings,  and  their  third  valencies  are  saturated 
by  hydrogen  atoms.  These  two  atoms  of  carbon  have 
still  each  one  valency  to  be  disposed  of;  the  fourth 
valency  seems  to  flutter  in  the  air.  Now  comes  in 
Kekule'  s  stroke  of  genius.  The  two  carbon  atoms  at 
the  ends  of  the  chain  dispose  of  their  fourth  available 
valency  between  themselves,  as  we  have  indicated 
above  by  a  broken  line.  These  atoms  no  longer  stand 


FORMULA  OF   BENZENE.  249 

at  the  ends  of  a  chain,  for,  as  the  following  scheme 
shows,  all  the  six  atoms  of  carbon  are  bound  together 
in  the  form  of  a  ring.  This  scheme  represents  all  the 
carbon  atoms,  and  also  all  the  hydrogen  atoms,  as 
behaving  in  the  same  way,  for  all  are  arranged  in 
exactly  similar  relative  positions. 


Formula  of  the  hydrocarbon  benzene. 


Kekule's  hypothesis  presents  a  ring-formed  com- 
bination of  the  six  carbon  atoms  ;  hence  the  expression 
"chemistry  of  rings" — that  is,  of  compounds  which  are 
supposed  to  contain  an  atomic  complex  whose  atoms 
are  arranged  in  a  ring.  As  it  is  somewhat  inconvenient 
to  draw  rings,  it  is  customary  to  retain  the  straight 
lines  between  the  carbon  atoms,  and  to  use  a  hexagon 
formula  in  place  of  a  ring.  As  everyone  who  is 
acquainted  with  the  subject  knows  that  the  carbon 
atoms  in  the  ring  are  alternately  singly  and  doubly 
linked — in  the  manner  shown  in  the  above  formulae — 
it  is  usual  to  omit  the  double  lines,  and  to  use  a 
hexagon  only.  We  constantly  meet  this  hexagon 
formula  in  those  books  which  are  concerned  with  the 
chemistry  of  compounds  that  contain  ring-formed 
atomic  complexes. 


250      INTRODUCTION   TO   MODERN   CHEMISTRY. 

The  possibility  of  replacing  any  monovalent  atom  or 
group  by  another  monovalent  atom  or  group,  etc.,  holds 
good  when  we  start  from  benzene.  If  one  atom  of 
hydrogen  in  benzene  is  replaced  by  chlorine,  we  obtain 
chlorobenzene.  Only  one  chlorobenzene  (C6H5C1)  can 
exist,  for  the  result  is  the  same  which  of  the  six  atoms 
of  hydrogen  is  replaced  by  an  atom  of  chlorine. 

C-C1 


H— C 


H— C 


C— H 


C— H 


C— H 

Chlorobenzene. 

But  if  two  atoms  of  hydrogen  in  benzene  are  re- 
placed by  two  atoms  of  chlorine,  it  is  evident  from 
the  formulae  that  three  isomeric  dichlorobenzenes  are 
possible.  These  three  modifications  are : — 

C— Cl  C— Cl  C-C1 


H—  Cf      X|C-C1 

H—  cr    xc—  H 

H-C7     X,C—  H 

ortho 

H—  Cl        1C—  H 

\/ 

meta 

H-cl          C-C1 

para 

H-C,        Jc—  H 

C—  H 

\/ 

C-H 

\/ 
C-C1 

(I) 
Dichlorobenzene. 

(II) 
Dichlorobenzene. 

(III) 

Dichlorobenzene. 

The  two  chlorine  atoms  in  the  first  dichlorobenzene 
are  attached  to  contiguous  carbon  atoms,  in  the  second 
they  are  attached  to  carbon  atoms  which  are  separated 
by  another  atom  of  carbon,  and  in  the  third  the  carbon 
atoms  whereto  the  chlorine  atoms  are  attached  are 
separated  by  two  other  atoms  of  carbon.  It  may  be 
seen  by  trying  that  other  modifications  are  impossible. 


DERIVATIVES   OF   BENZENE.  251 

Suppose,  for  example,  that  the  second  chlorine  atom 
in  III  is  attached  to  the  next  carbon  atom  to  the  left. 
We  get  the  same  arrangement  as  is  presented  in  II ; 
for  we  get  an  arrangement  wherein  the  two  chlorine 
atoms  are  attached  to  carbon  atoms  that  are  separated 
by  one  other  atom  of  carbon. 

Diderivations  of  benzene  of  the  kind  shown  in  I 
are  called  ortho  compounds,  diderivations  of  the  kind 
shown  in  II  are  called  meta  compounds,  and  dideriva- 
tions of  the  kind  shown  in  III  are  called  para  com- 
pounds. We  speak  of  orthodichlorobenzene,  meta- 
dichlorobenzene,  and  paradichlorobenzene. 

What  holds  good  when  we  are  dealing  with  two 
atoms  of  chlorine  also  holds  good  for  any  other  two 
monovalent  atoms  or  atomic  groups  which  may  be 
brought  into  the  benzene  ring. 

Before  the  promulgation  of  Kekule's  theory,  no 
reason  could  be  given  for  the  existence  of  three 
isomeric  diderivatives  such  as  the  dichlorobenzenes, 
and  no  reason  could  be  given  why  there  should  not 
be  four  or  five  such  derivatives  of  benzene.*  Now 
it  is  all  self-evident.  The  number  of  diderivatives  of 
benzene  known  to-day  is  very  great,  but  of  none 
have  more  than  three  isomeric  forms  been  obtained, 

*  A  model  may  be  used,  in  place  of  the  hexagon  formula,  to 
express  Kekule's  hypothesis ;  black  balls  may  represent  the 
carbon  atoms,  white  balls  the  hydrogen  atoms,  and  so  on,  and 
the  joining  lines  may  be  made  with  pieces  of  wire.  The  various 
consequences  that  flow  from  the  valencies  of  the  different  atoms, 
etc.,  may  be  demonstrated  by  the  aid  of  such  a  model,  in  the  way 
indicated  by  von  Baeyer  in  his  lecture,  delivered  at  the  Kekule 
anniversary,  on  the  use  of  such  models  in  the  development  ot 
theoretical  conceptions  in  chemistry.  Such  models  show  to  some 


252      INTRODUCTION    TO   MODERN   CHEMISTRY. 

notwithstanding  the  endless  trouble  that  has  been  taken 
to  prepare  more ;  hence  this  consequence  of  the  theory 
of  Kekule  has  become  one  of  the  main  supports  of  that 
theory.  The  theory  declares  with  certainty  what 
nature  allows  and  what  she  does  not  allow.  In  speak- 
ing of  the  incompleteness  of  science,  it  is  sometimes 
said,  half  in  fun,  that  "  the  exception  proves  the  rule  "  ; 
but  this  is  one  of  those  rules  to  which  not  a  single 
exception  has  been  observed. 

Nitrobenzene  is  obtained  by  treating  benzene  with 
nitric  acid.  In  this  reaction  there  is  not  produced  a 
salt-like  compound,  such  as  is  formed,  for  example,  by 
the  reaction  of  nitric  acid  on  cellulose  (see  p.  184), 
but  a  true  7«M?-compound.  The  group  NO2  is  called 
the  nitrogroup.  The  nitro-compounds  are  not  com- 
parable with  the  salts  of  nitric  acid ;  in  these  compounds 
nitric  acid  has  lost  its  hydroxyl  group,  OH. 

C-H  C— H 


H— C 
H-C 


C— Hi  H— C 

i     +OH|-N02    = 
C— !H  H— C 


C— H  C— H 

Benzene        +  nitric  acid    =  nitrobenzene  +  water. 

C6H6  +     HO— NO2     -  C6H5— NO2     +    H2O. 

extent  that  what  Hertz  (the  discoverer  of  electric  waves)  said  of 
Clerk  Maxwell's  electromagnetic  theory  of  light  is  true  also 
of  Kekule's  theory  : — "  One  cannot  study  this  wonderful  theory 
without  feeling  at  times  as  if  mathematical  formulae  had  a  life 
of  their  own,  as  if  there  was  in  them  a  special  understanding,  as 
if  they  were  more  subtle  than  we  are,  more  subtle  than  their 
discoverer,  as  if  they  gave  us  back  more  than  was  put  into  them 
at  the  time.'' 


DERIVATIVES   OF   BENZENE.  253 

In  the  formation  of  a  salt  or  a  salt-like  compound 
from  nitric  acid  the  hydrogen  atom  of  the  hydroxyl 
group  of  the  acid  is  replaced  by  a  metal,  as  in  the 
formation  of  potassium  nitrate,  NO2 — OK,  or  by  a 
monovalent  organic  radicle,  as  in  the  formation  of 
methyl  nitrate,  NO2 — O(CH3).  All  such  compounds 
contain  the  group  NO3  (or  NO2 — O — ),  which  is  the 
monovalent  radicle  of  nitric  acid  when  hydrogen  has 
been  removed.  The  nitro-compounds,  on  the  other 
hand,  contain  another  monovalent  radicle  of  nitric 
acid,  namely  NO2 — ,  which  is  nitric  acid  wherefrom 
hydroxyl  has  been  removed.  Nitric  acid  can  be 
obtained  from  the  salts  of  that  acid  by  their  reactions 
with  a  stronger  acid  (compare  p.  173)  ;  but  the  acid 
cannot  be  obtained  in  that  way  from  the  nitro- 
compounds. 

If  we  replace  an  atom  of  hydrogen  in  benzene  by 
the  monovalent  radicle  of  ammonia — by  NH2 — we  ob- 
tain the  well-known  compound  amidobenzene,  or  aniline, 
the  compound  which  gives  its  name  to  the  modern 
coal-tar  colour  industry. 

C-H 


H»C 


H— C 


C— NH., 


C— H 


C— H 

Aniline. 


Benzene,  which  is  the  starting  point  of  our  considera- 
tions, the  mother-substance  of  all  ring-formed  atomic 
arrangements,  besides  being  obtained  from  benzoic 


254      INTRODUCTION    TO   MODERN    CHEMISTRY. 

acid  and  many  other  compounds,  is  found  in  coal-tar. 
When  this  black  waste  product  of  the  gas-works  is 
distilled,  clear  oil  passes  over :  this  oil  contains  ben- 
zene, besides  carbolic  acid,  naphthalene,  and  many 
other  useful  substances.  The  benzene  is  purified  by 
fractional  distillation  (see  p.  201) — it  boils  at  80°  C. 
[176°  F.] — and  is  converted  into  aniline.  The  fol- 
lowing method  is  employed  for  doing  this.  First  of 
all  the  benzene  is  transformed  into  nitrobenzene,  and 
the  nitrobenzene  is  then  changed  to  aniline  by  the 
reaction  with  it  of  nascent  hydrogen  (see  p.  107). 

Colours  are  obtained  from  aniline  under  all  sorts  of 
conditions.  To  mention  one  set  of  conditions  only : 
if  aniline  is  mixed  with  methyl  aniline  and  the  mixture 
is  heated  with  substances  very  rich  in  oxygen,  a  pro- 
cess of  oxidation  occurs,  and  a  red  mass  is  obtained, 
which  is  fuchsine: — 

2C6H5NH2  +  C6H4(CH3)NH2  +    3O      =  C19H19N3O   +   2H2O. 
Aniline      -r     methyl  aniline     +  oxygen  =      fuchsine      +    water. 

If  an  atom  of  hydrogen  in  benzene  is  replaced  by 
hydroxyl,  OH,  we  obtain  phenol,  C6H6 — OH.  This 
compound  is  generally  called  carbolic  acid,  which  name 
was  given  to  it  by  its  discoverer  in  the  thirties  of  the 
nineteenth  century.  The  expanded  formula  of  this 

compound  is  : — 

C— H 


H— C 


H— C 


C— OH 


C— H 


C-  H 

Phenol,  or  carbolic  acid. 


PYRIDINE.  255 

The  name  carbolic  acid  implies  that  the  compound 
reacts  as  an  acid  and  forms  salts — that  the  hydrogen 
of  the  hydroxyl  group  is  replaceable  by  metals.  Car- 
bolate  of  calcium,  carbolate  of  silver,  etc.,  have  been 
prepared.  The  atom  of  carbon  whereto  the  hydroxyl 
group  is  attached  in  this  compound  is  connected 
with  the  other  carbon  atoms  in  a  way  which  is  very 
different  from  that  of  the  lin kings  in  those  open 
chains  of  carbon  atoms  which  yield  true  alcohols 
that  do  not  show  any  acidic  properties.  The  differ- 
ences between  the  behaviours  of  the  hydroxyl  groups 
in  C6H5OH  and  in  the  true  alcohols  is  connected 
with  the  ring  formation  of  the  carbon  atoms  in 
phenol. 

In  the  eighties  of  the  nineteenth  century  chemists 
had  slowly  come  to  the  conviction  that  a  ring-formed 
atomic  complex  need  not  necessarily  be  formed  of 
CH  groups,  as  is  the  case  with  benzene.  Each  of  the 
carbon  atoms  that  form  the  ring  is  connected  to  other 
carbon  atoms  by  three  valencies — by  two  valencies  on 
one  side  and  by  one  on  the  other  side — while  the 
fourth  valency  of  each  is  satisfied  by  an  atom  of 
hydrogen.  Now  the  atom  of  nitrogen  is  trivalent ; 
and  it  was  found  that  such  a  trivalent  atom  was  able 
to  close  the  ring  by  taking  the  place  of  the  trivalent 
group  CH.  No  atom  of  hydrogen  can  be  attached 
to  the  atom  of  nitrogen,  inasmuch  as  the  three 
valencies  of  that  atom  are  required  for  closing  the 
ring. 

The  arrangement  of  the  atoms  in  a  molecule  of  the 
body  we  are  considering  will  be  as  follows : — 


256      INTRODUCTION    TO    MODERN    CHEMISTRY. 

C— H 


H-C 


H— C 


C— H 


C— H 


N 
Pyridine. 

This  compound,  pyridine,  can  be  obtained  from  certain 
alkaloids.  But  what  are  alkaloids!  Methods  have 
long  been  known  for  obtaining  indifferent  substances 
from  plants — substances  which  do  not  change  vegetable 
colouring  matters,  and  are  neither  acids  nor  bases ; 
we  recall,  for  instance,  starch,  sugar,  and  the  like. 
Acids,  such  as  citric,  malic,  and  tartaric  acid,  were  also 
prepared  from  plants.  But  the  fact  was  long  over- 
looked that  many  plants  contain  substances  which  have 
an  alkaline  reaction,  colour  red  litmus  blue,  and  com- 
bine with  acids  to  form  salts.  It  was  in  1817  that  the 
existence  of  such  substances  was  finally  proved  by  the 
preparation  of  the  alkaline  compound  morphine  from 
opium.  Attention  being  thus  called  to  the  subject 
several  substances  resembling  morphine  were  soon 
obtained — for  instance,  quinine,  com'me,  veratrine,  etc. 
The  very  convenient  specific  name  alkaloid  was  applied 
to  all  these  "  alkalis  from  plants  "  :  the  name  implies  a 
compound,  obtained  from  plants,  which  reacts  like  an 
alkali.  The  investigation  of  the  alkaloids  has  shown 
that,  with  few  exceptions,  they  are  derivatives  of 
pyridine.  But  pyridine  itself  can  be  prepared  from 
coal-tar  (see  forward) ;  hence  it  can  be  obtained  in 
abundance. 

In   the  sense  in  which  we  spoke  of  a  chemistry  of 


ALKALOIDS.  257 

benzene  we  may  also  speak  of  a  chemistry  of  pyridine  • 
that  is  to  say,  the  hydrogen  atoms  in  the  molecule  of 
pyridine  may  be  replaced  by  radicles  of  different  kinds, 
and  a  great  number  of  compounds  may  be  obtained. 
The  alkaloids  are  among  the  most  important  compounds 
derived  from  pyridine.  The  study  of  certain  individual 
alkaloids  has  advanced  so  far  that  the  relative  arrange- 
ments of  all  the  atoms  in  their  molecules  have  been 
worked  out.  In  such  cases  it  is  known  how  many  methyl 
or  other  groups  have  taken  the  places  of  hydrogen 
atoms  in  pyridine ;  and  several  alkaloids  have  been 
made  artificially  in  the  laboratory  by  introducing  the 
proper  atomic  groups  into  the  pyridine  molecule.  The 
alkaloids  from  plants  usually  act  very  energetically  on 
the  bodies  of  animals  and  human  beings ;  many  are 
strong  poisons,  such  as  strychnine  and  morphine.  But 
these  poisonous  substances  often  act  as  most  important 
remedies  when  they  are  administered  in  extremely 
small  doses.  Quinine,  especially,  has  been  used  for 
many  years  as  a  medicine,  because  it  lowers  the  bodily 
temperature  during  fever;  and  as  fever  accompanies 
many  kinds  of  illness,  quinine  may  be  employed  in 
very  different  ailments. 

The  chemical  study  of  quinine  is  not  yet  completed. 
The  empirical  formula  of  quinine,  C2oH24N2O2,  shows 
that  the  molecule  of  this  compound  is  composed  of 
forty-eight  atoms.  The  arrangement  of  these  atoms, 
which  is  very  complicated,  has  not  yet  been  fully 
elucidated — we  do  not  yet  know  how  the  atoms  are 
linked  to  one  another  in  the  molecule.  So  long  as  the 
arrangement  of  the  atoms  is  unknown,  it  is  not  possible 
to  make  quinine  artificially;  for  how  can  we  link  together 

17 


258      INTRODUCTION    TO   MODERN   CHEMISTRY. 

atoms  as  they  are  linked  in  quinine  if  we  do  not  know 
what  the  linkings  are  in  the  molecule  of  that  compound  ? 
Nevertheless,  the  many  investigations  that  have  been 
made  of  pyridine  derivatives  have  led  to  the  recogni- 
tion of  bodies  in  whose  molecules  atoms  and  atomic 
groups  are  linked  together  in  such  a  way  that  these 
bodies  most  probably  resemble  the  natural  alkaloids  ; 
and  experiments  made  on  animals  in  a  state  of  fever 
have  shown  that  these  artificially  made  preparations 
lower  the  temperature.  In  this  way  we  have  been  able 
to  make  artificial  febrifuges,  such  as  antipyrine,  which 
are  useful  in  the  illnesses  of  human  beings. 

We  have  now  gained  some  conception  of  "  organic 
chemistry."  We  have  become  acquainted  with  the 
two  classes  of  compounds  which  are  considered  in  that 
branch  of  chemistry — those  wherein  the  carbon  atoms 
are  arranged  in  open  chains,  and  those  wherein  the 
arrangement  is  ring-formed.  These  two  classes  contain 
all  the  compounds  of  carbon. 


THE  CHEMISTRY  OF  ORGANISED  SUBSTANCES. 

Organic  chemistry  is  not  to  be  confused  with  the 
chemistry  of  organised  substance^ — that  is,  those  sub- 
stances which  make  life  possible,  and  in  which  and  with 
the  help  of  which  life  proceeds.  ,We  already  know  that 
those  organised  bodies  which  contain  nitrogen  and  have 
a  very  complex  composition  are  classed  as  albuminoids. 
Crystallisation  is  the  best  way  of  purifying  solid  sub- 
stances. But  only  very  few  albuminoids  can  be  crystal- 


ALBUMINOIDS.  259 

.  lised,  and  so  obtained  completely  pure.  The  red 
colouring  substance  in  blood  is  an  albuminoid  ;  it  is 
called  oxy haemoglobin.  The  oxyhaemoglobin  from  human 
blood  crystallises  with  great  difficulty;  it  is  scarcely 
possible  to  purify  this  body  thoroughly  by  crystallisa- 
tion :  but  oxyhaemoglobin  from  the  blood  of  horses 
may  be  crystallised,  and  so  purified,  readily.  An 
analysis  of  oxyhaemoglobin  leads  to  the  formula 
Q55H881N149S2O177Fe.  From  what  we  know  of  organic 
chemistry,  we  can  understand  that  there  is  practically 
no  hope  of  our  being  able  to  find  out  how  these  1765 
atoms  are  linked  together.  Even  supposing  that  this 
question,  which  seems  insolvable,  were  solved  in  the 
course  of  time,  we  should  still  have  to  face  the  ex- 
tremely difficult  task  of  properly  linking  together  all 
the  atoms  and  atomic  groups  present  in  oxyhaemo- 
globin in  order  to  prepare  that  substance  artificially. 
Most  of  the  albuminoids  do  not  crystallise ;  hence 
but  few  of  the  bodies  can  be  obtained  chemically  pure 
in  a  satisfactory  way :  this  makes  the  problems  of 
determining  their  constitution  and  synthesising  them 
quite  insolvable  at  present.  What  knowledge  we  have 
seems  to  indicate  that  these  problems  will  never  be 
solved. 

The  Asymmetric  Carbon  Atom. 

If  we  inquire  in  what  manner  the  four  hydrogen  atoms  in  the 
molecule  of  methane  are  arranged  in  space  around  the  atom 
of  carbon,  we  must  suppose  that  these  four  atoms  are  placed 
symmetrically  about  the  carbon  atom.  Such  an  arrangement  is 
obtained  if  we  think  of  the  four  hydrogen  atoms  as  joined  by 
lines  that  form  the  sides  of  triangles,  and  of  a  pyramid  formed 
of  four  equal  triangles,  with  the  atom  of  carbon  in  the  middle. 


260      INTRODUCTION    TO   MODERN    CHEMISTRY. 

As  the  four  atoms  of  hydrogen  are  grouped  quite  symmetrically 
as  regards  the  atom  of  carbon,  we  obtain  a  regular  pyramid, 
that  is,  a  tetrahedron.  Now,  if  one  of  the  hydrogen  atoms  is 
replaced  by  something  else,  the  attractive  force  exerted  by  the 
atom  of  carbon  will  cause  the  replacing  atom  (or  group)  to 
occupy  a  position  not  exactly  the  same  as  that  which  was  occu- 
pied by  the  hydrogen  atom  that  has  been  replaced.  If  we  think 
of  this  replacing  atom  (or  group)  as  one  of  the  summits  of  the 
tetrahedron,  the  other  three  summits  being  atoms  of  hydrogen, 
then  the  arrangement  is  no  longer  exactly  symmetrical ;  it  is  no 
longer  a  tetrahedron.  If  an  atom  of  carbon  is  united  to  four 
different  atoms  or  atomic  groups,  the  pyramid  becomes  quite 
irregular ;  it  is  asymmetric.  An  atom  of  carbon  thought  of  as 
so  situated  is  called  an  asymmetric  carbon  atom.  The  name  is 
rather  infelicitous,  for  all  carbon  atoms  are,  of  course,  always 
identical.  It  is  not  the  carbon  atom  that  is  asymmetrical  in  the 
case  we  are  considering  ;  it  is  the  positions  of  the  four  atoms 
or  groups  (which  may  be  called  A,  B,  C,  and  D),  relatively 
to  the  atom  of  carbon  whereto  they  are  bound,  that  are 
asymmetrical.  Such  asymmetric  pyramids  can  exist  in  two 
forms,  that  are  alike  in  all  respects  save  that  one  bears  to  the 
other  the  relation  that  an  object  bears  to  its  image  in  a  mirror, 
and  the  two  cannot  be  superimposed  on  one  another.  The 
relation  between  the  two  is  the  same  as  that  between  a  right- 
hand  and  a  left-hand  glove.  The  following  figures  show  such  a 
right-handed  pyramid  and  its  left-handed  counterpart : — 


Now,  many  chemical  compounds  possess  what  seems  to  be  the 
accidental  property  of  rotating  the  plane  of  polarisation  of  a  ray 
of  light.  These  compounds  act  in  a  certain  definite  way  on 
what  is  called  polarised  light  when  solutions  of  them  are 
examined  in  a  polarisation-apparatus.  It  would  seem  as  if  there 
could  be  no  connection  between  the  arrangement  of  atoms  in 
molecules  and  polarised  light.  But  this  assumption  is  incorrect ; 


ASYMMETRIC   CARBON    ATOMS,  26 1 

for  it  has  been  found  that  solutions  of  carbon  compounds  that 
contain  an  asymmetric  carbon  atom  or  several  asymmetric 
carbon  atoms  are  able  to  act  on  polarised  light.  Lactic  acid 
is  a  fairly  simple  example  of  such  a  compound.  Although 
methods  are  known  whereby  many  compounds  have  been  pre- 
pared in  the  laboratory  containing  asymmetric  carbon  atoms 
and  rotating  the  plane  of  polarisation  of  a  ray  of  light,  never- 
theless it  is  at  present  impossible  even  to  think  of  applying  such 
complicated  methods  to  so  complex  a  substance  as  oxyhaemo- 
globin.  In  a  word,  this  apparently  secondary  property  seems  to 
make  impossible  the  artificial  preparation  of  an  oxyhsemoglobin 
which  should  be  the  same  as  the  natural  product. 

Lactic  acid  can  be  obtained  both  from  milk  and  the  flesh  of 
animals.  The  empirical  formula  of  this  acid  is  C3HGO3.  The 
lactic  acids  prepared  by  different  methods  are  not  identical,  but 
isomeric  in  a  certain  sense.  While  lactic  acid  from  milk  does 
not  act  on  polarised  light,  the  acid  from  flesh  rotates  the  plane 
of  polarisation  in  a  right-handed  direction.  A  close  examination 
of  sarcolactic  acid  has  shown  that  the  arrangement  of  the  atoms 
in  the  molecule  of  this  compound  is  such  that  an  asymmetric 
carbon  atom  comes  between  the  two  other  carbon  atoms.  The 
following  is  the  expanded  formula  of  the  compound  : — 

H      H 

I        I        ^O 
H— C— C-C^ 

I       I        ^OH 
H    OH 

The  C  is  the  asymmetric  atom  ;  for  the  four  valencies  of  this 
atom  are  saturated,  one  by  the  group  CH3,  another  by  an  atom 
of  hydrogen,  another  by  the  group  OH,  and  the  fourth  by  the 
group  COOH.  This  sarcolactic  acid  has  been  prepared  arti- 
ficially, and  the  product  of  this  synthesis  exerts  the  same 
influence  on  polarised  light  as  the  naturally  occurring  acid. 
However  remarkable  is  this  synthesis,  yet  it  is  easy  compared 
with  the  synthesis  of  the  complex  of  1765  atoms  of  oxyhasmo- 
globin,  wherein  there  may  be  perhaps  300  asymmetric  carbon 
atoms,  some  of  which  may  cause  right-handed  rotation  and 
others  left-handed  rotation  of  the  plane  of  polarisation  of  a  ray 
of  light. 


262      INTRODUCTION    TO    MODERN    CHEMISTRY. 

We  have  still  to  deal  with  the  two  oxides  of  the 
element  carbon,  compounds  which  it  is  not  customary 
to  class  as  organic.  We  shall  also  consider  coal-gas, 
and,  finally,  we  shall  say  a  little  about  acetylene  gas. 
Carbon  combines  with  oxygen  in  two  proportions  only  : 
one  atom  of  carbon  combines  with  one  atom  of  oxygen 
to  form  carbon  monoxide  gas,  CO,  and  with  two  atoms 
of  oxygen  to  form  carbon  dioxide  gas,  CO2.  The  second 
of  these  compounds  is  also  called  carbonic  anhydride, 
and  also,  although  not  quite  correctly  (see  p.  49), 
carbonic  acid  gas,  or  carbonic  acid. 

Carbon  monoxide  is  the  only  compound  wherein  the  carbon 
atom  is  not  tetravalent.  For,  as  the  oxygen  atom  is  divalent, 
and  one  atom  of  carbon  holds  only  one  atom  of  oxygen  in  the 
molecule  of  this  compound,  the  carbon  atom  is  only  divalent  in 
C  ~O.  In  all  the  vast  number  of  carbon  compounds,  with  this 
single  exception,  the  carbon  atom  is  tetravalent.  This  exception 
must  be  accepted  as  a  fact  by  itself;  we  cannot  attempt  to  give 
an  explanation  of  it  in  this  place.  Carbon  monoxide  impresses 
us  as  "an  unsaturated  compound":  for  example,  if  carbon 
monoxide  gas  and  chlorine  gas  are  mixed,  they  combine  to  form 
carbon  oxychloride,  in  which  compound  the  carbon  atom  is 
tetravalent. 

CO  +  C12  °~C<<C\ 

Carbon  monoxide      4-        chlorine        =       carbon  oxychloride. 

Carbon  monoxide  gas  is  formed  when  coal  or 
charcoal  burns  in  a  limited  supply  of  air,  that  is, 
when  there  is  not  enough  oxygen  to  burn  the  fuel 
completely.  Hence  this  gas  issues  from  badly  con- 
structed stoves,  and  especially  when  such  stoves  are 
full  of  red  hot  fuel  and  the  valves  are  closed.  Carbon 
monoxide  is  very  poisonous,  and  fatal  accidents  have 


ACTION   OF   CARBON    MONOXIDE   ON    BLOOD.   263 

been  caused  by  the  escape  of  this  gas  into  rooms. 
The  poisonous  action  of  carbon  monoxide  depends  on 
the  following  considerations.  We  have  become  ac- 
quainted with  oxyhaemoglobin,  the  red  colouring  matter 
of  blood.  The  prefix  oxy  denotes  that  this  substance 
is  rich  in  oxygen  ;  it  is  this  compound  which,  in  the 
lungs,  takes  the  oxygen  from  the  air  (see  p.  132). 
Now,  if  the  air  of  a  room  contains  carbon  monoxide 
gas,  this  gas  also  is  absorbed  by  the  red  colouring 
matter  of  the  blood,  and  carbon  oxide  haemoglobin  is 
formed,  besides  oxyhaemoglobin.  While  the  oxyhaemo- 
globin that  circulates  in  the  blood  is  ready  to  part  with 
oxygen  to  the  body — constantly  taking  up  oxygen 
again  in  the  lungs  in  place  of  that  which  it  has  given 
to  the  body — carbon  oxide  haemoglobin,  on  the  con- 
trary, is  a  very  stable  compound,  and  it  can  be  de- 
composed only  very  slowly  by  the  body  ;  hence  the 
quantity  of  this  compound  in  the  blood  is  increased 
by  every  breath  that  is  taken  in  an  atmosphere  which 
contains  carbon  monoxide.  When  there  is  a  certain 
amount  of  this  compound  in  the  blood,  the  vital 
functions  of  the  blood  can  no  longer  be  exercised, 
and  death  results.  The  stability  of  the  compound  of 
carbon  monoxide  with  haemoglobin  is  shown  by  the 
fact  that  it  can  be  detected  in  blood  from  a  corpse 
several  months  after  death.  It  is,  therefore,  easy  to 
determine  whether  death  has  been  caused  by  carbon 
monoxide,  a  matter  which  is  sometimes  of  legal  im- 
portance. Ordinary  blood  is  wholly  decomposed  in 

the  course  of  a  few  months. 

» 

The  preparation  of  carbon  monoxide  gas  by  burning 


264      INTRODUCTION    TO   MODERN   CHEMISTRY, 

charcoal  in  a  limited  supply  of  air  is  not  a  convenient 
laboratory  method  for  making  that  compound.  The 
product  of  the  reaction  is  a  mixture  of  carbon  mon- 
oxide with  the  nitrogen  of  the  air  wherein  the  carbon 
has  been  burnt.  We  shall  prepare  the  gas  by  another, 
very  convenient,  method. 

Oxalic  acid  is  an  organic  acid;  it  contains  carbon. 
Sorrel  plants  contain  salts  of  oxalic  acid,  from  which  the 
acid  may  be  prepared  ;  but  the  process  now  employed 
consists  in  fusing  caustic  soda  with  sawdust,  whereby 
the  sodium  salt  of  oxalic  acid  is  produced.  The  acid 
is  very  cheap :  its  chief  applications  are  in  the  colour 
industries.  The  formula  of  oxalic  acid  is  H2C2O4.  If 
this  acid  is  treated  with  reagents  that  withdraw  water 
— with  concentrated  sulphuric  acid,  for  instance  (which, 
as  we  know,  is  a  strong  dehydrating  substance) — water 
is  formed,  and  there  remain  two  atoms  of  carbon  and 
three  atoms  of  oxygen.  This  reaction  may  be  ex- 
pressed as  follows,  using  an  expanded  formula  and 
indicating  the  decomposition  by  a  dotted  line  :  — 


O~_C—  O— |H 
OUC— !O— H 


The  group  of  five  atoms  (C2O3)  falls  to  pieces, 
forming  a  molecule  of  carbon  monoxide,  CO,  and  a 
molecule  of  carbon  dioxide,  CO2.  To  put  the  matter 
more  briefly  :  when  a  mixture  of  oxalic  acid  and  sul- 
phuric acid  is  warmed,  the  oxalic  acid  decomposes 
to  water,  which  is  held  by  the  sulphuric  acid,  and  a 
mixture  of  the  two  gases  carbon  monoxide  and  carbon 


PREPARATION   OF   OXIDES   OF   CARBON.        265 

dioxide,  which  issues  from  the  flask  wherein  the  opera- 
tion is  conducted. 

H2C.A    =          CO  +        CO,          +  H2O. 

Oxalic  acid  =  carbon  monoxide  +  carbon  dioxide  +  water. 

If  the  stream  of  gas  is  led  through  a  solution  of 
caustic  potash  in  a  washing-bottle,  the  carbon  dioxide 
is  retained  by  the  alkali,  wherewith  it  forms  potassium 
carbonate,  and  pure  carbon  monoxide  gas  passes  on, 
and  may  be  collected  in  a  cylinder  over  water. 

CO.,     +       CO      +2KOH=     K,CO3    +  H2O  +      CO. 
Carbon          carbon  ,     ,        potassium  carbon 

dioxide   +  monoxide  +  POtash  =  carbonate  +  water  +  monoxide. 

By  bringing  a  light  to  a  jarful  of  carbon  monoxide, 
we  see  that  this  gas  is  combustible,  and  that  it  burns 
with  a  bluish  and  very  slightly  luminous  flame.  The 
product  of  combustion  is  carbonic  acid  gas. 

CO          +      O     =        CO2. 
Carbon  monoxide  +  oxygen  =  carbon  dioxide. 

If  carbon  is  burnt  in  much  air,  or,  as  we  have  done 
already  (p.  1 1 8),  in  pure  oxygen  gas,  one  atom  of  carbon 
combines  with  two  atoms  of  oxygen,  and  carbon  dioxide 
(or  carbonic  anhydride)  is  produced.  The  carbon  atom 
in  this  molecule  is  tetravalent,  as  two  divalent  atoms 
of  oxygen  are  held  by  one  atom  of  carbon,  O~C— O. 

Carbon  dioxide  is  not  prepared  in  the  laboratory  by 
burning  carbon  in  air  :  the  product  of  this  reaction 
would  contain  the  whole  of  the  nitrogen  that  was 
present  in  the  air  used  for  burning  the  carbon.  There 
is  no  difficulty  in  preparing  the  pure  compound  by 


266      INTRODUCTION    TO    MODERN    CHEMISTRY. 

another  method.  Carbon  dioxide  is  readily  driven 
out  of  its  salts  by  the  action  of  the  stronger  acids. 
Carbonates,  especially  calcium  carbonate,  are  found 
plentifully  in  nature.  Limestone  is  impure  calcium 
carbonate  ;  marble  is  a  purer  form  of  the  same  com- 
pound. If  pieces  of  marble  are  placed  in  a  Kipp's 
apparatus  and  hydrochloric  acid  is  allowed  to  mix 
with  the  marble  (in  the  manner  described  on  p.  35),  a 
regular  stream  of  carbon  dioxide  gas  is  obtained  by 
simply  opening  the  stopcock  of  the  apparatus.  The 
other  product  of  the  reaction  —  calcium  chloride  —  is 
very  soluble  in  water  ;  it  therefore  remains  dissolved 
in  the  aqueous  solution  of  hydrochloric  acid  used  in 
the  process.  The  following  equation  expresses  the 
decomposition  :  — 

CaCO3      +       2HC1         =     CO.,     +    CaCl,    +   H,O. 
Calcium  hydrochloric   _   carbon        calcium 

carbonate     "  acid  ~  dioxide   *  chloride    ~* 

When  an  acid  is  set  free  from  its  salts,  the  acid 
usually  contains  hydroxyl  groups  :  we  recall  the  case 
of  sulphuric  acid.  In  the  present  case  the  hypothetical 
carbonic  acid,  H2CO3,  which  we  should  expect  to 
obtain,  decomposes  into  carbonic  anhydride  and  water. 

OH 
The   true  carbonic  acid,   COu,  which   we  should 


expect  to  be  formed  in  accordance  with  the  reaction 
CaCO3  +  2HC1  =  H2CO3  +  CaCl2,  has  not  been  iso- 
lated. We  cannot  attempt  an  explanation  why  this 
is  so  ;  the  fact  must  be  accepted.  We  shall  return 
to  carbonic  acid  when  we  are  dealing  with  the  formulae 
of  the  carbonates  of  the  light  metals. 


FERMENTATION    OF   SUGAR.  267 

As  has  been  already  noticed,  carbon  dioxide  is  the 
gas  which  is  contained  in  all  effervescing  fermented 
liquors — in  beer  and  champagne,  for  instance.  The 
carbon  dioxide  in  these  liquors  is  formed  by  the 
fermentation  of  the  sugar  which  is  a  necessary  in- 
gredient of  all  liquids  capable  of  being  fermented.  The 
formula  of  this  sugar,  called  grape  sugar  by  chemists, 
is  C0H12O(i.  Yeast  consists  of  small  cells  which,  like 
the  cells  of  moulds,  are  always  present  in  the  air.  If 
yeast  is  present  in  a  liquid  that  also  contains  grape 
sugar  (and  as  yeast-cells  are  always  in  the  air,  they 
readily  fall  into  any  liquid  that  is  exposed  to  the  air), 
the  yeast  grows  rapidly  in  the  liquid,  and  decomposes 
the  sugar,  in  accordance  with  the  equation  : — 

C6H,A  2C,H60       +  2CO,. 

Grape  sugar     =         alcohol          +      carbon  dioxide. 

This  process  is  called  fermentation.  All  fermented 
drinks,  then,  contain  alcohol.  The  carbon  dioxide  that 
is  produced  in  the  fermentation  escapes  into  the  air 
when  the  fermentation  proceeds  in  open  vessels,  as  in 
making  wine ;  but  this  gas  is  retained  in  such  drinks 
as  beer  and  champagne.  Artificial  seltzer  water  and 
other  lt aerated  liquids"  are  charged  with  carbon 
dioxide,  which  is  generally  made  from  marble  and 
hydrochloric  acid,  by  pumping  in  the  gas  under 
pressure. 

As  carbon  and  oxygen  combine  to  form  carbon 
dioxide,  CO2,  so  do  carbon  and  sulphur  combine,  when 
they  are  heated  together,  to  form  carbon  disulphide, 
which  is  a  liquid  that  boils  at  45°  C.  [113°  F.].  This 


268      INTRODUCTION    TO    MODERN    CHEMISTRY. 

reaction  is  another  instance  of  the  chemical  likeness  of 
sulphur  and  oxygen.  The  equation  which  expresses 
the  combination  of  carbon  and  sulphur  is  as  follows  : — 

C        +        S2        =  CS2. 

Carbon      +      sulphur     =     carbon  disulphide. 

Our  reason  for  especially  mentioning  carbon  disul- 
phide is  the  following.  We  have  seen  that  all  the 
compounds  of  organic  chemistry,  and,  in  the  final 
analysis,  also  the  compounds  which  form  the  substance 
of  living  things,  can  be  referred  to  and  derived  from 
the  hydrocarbon  methane,  CH4.  Hence  it  is  of  especial 
interest  to  prepare  this  compound,  to  which  all  organic 
substances  are  referable,  from  inorganic  materials,  and 
so  to  find  a  way  of  passing  from  the  inorganic  to  the 
organic.  Of  the  many  processes  for  preparing  methane, 
the  following  is  that  which  enables  us  to  make  that 
compound  from  its  elements — that  is  to  say,  from 
inorganic,  non-living  materials.  As  we  know,  carbon 
disulphide  is  obtained  by  directly  combining  the  two 
elements  carbon  and  sulphur.  Sulphuretted  hydrogen 
also  can  be  made  from  inorganic  substances.  The 
preparation  of  this  compound  from  its  elements  is 
already  known  to  us  :  it  is  only  necessary  to  melt 
together  sulphur  and  iron,  and  to  pour  hydrochloric 
acid  on  the  iron  sulphide  that  is  so  formed  (compare 
p.  144).  Now,  if  a  mixture  of  carbon  disulphide  and 
sulphuretted  hydrogen  is  passed  over  red  hot  copper 
(just  as  we  passed  water-vapour  over  red  hot  iron, 
on  p.  31),  the  hoi  copper  lays  hold  of  the  sulphur  of 
both  compounds,  sulphide  of  copper  is  produced,  and 
the  carbon  which  was  combined  with  sulphur  in  one 


DRY   DISTILLATION   OF   COAL.  269 

of  the  compounds  combines  with  the  hydrogen  which 
was  united  to  sulphur  in  the  other  compound,  thereby 
forming  the  hydrocarbon  methane,  CH4.  The  following 
equation  expresses  the  reaction  : — 

CS,       +         2H.,S         +      8Cu      -       CH4        +    4Cu2S. 

Carbon          sulphuretted  =     methane  copper 

disulphide  hydrogen  sulphide. 

In  this  reaction,  then,  we  find  what  we  are  seek- 
ing— a  means  of  passing  from  inorganic  to  organic 
substances. 


THE  MANUFACTURE  OF  COAL-GAS. 

When  any  sort  of  material  is  placed  in  a  retort,  or 
in  a  closed  vessel  (which  need  not  necessarily  take  the 
form  of  a  retort),  and  is  subjected  to  the  action  of  a 
source  of  heat  outside  the  vessel,  any  substances  con- 
tained in  the  material  which  are  volatilised  by  heat, 
or  any  volatile  substances  which  are  produced  by  the 
action  of  heat  on  the  material,  are  driven  out,  and 
escape  from  the  retort  in  the  form  of  gases.  Such  a 
process  is  called  dry  distillation. 

The  dry  distillation  of  coal,  for  the  purpose  of 
making  coal-gas,  is  conducted  on  a  very  large  scale. 
The  coal  is  shot  into  tubular  vessels  made  of  fire-clay, 
which  are  placed  in  a  furnace  where  they  can  be 
heated  to  redness.  Although  the  coal  is  raised  to  a 
very  high  temperature,  it  cannot  burn  in  the  retorts, 
because  of  the  absence  of  air. 

Coal  is  what  is  left  of  very  ancient  forests,  the  wood 
of  which  has  been  gradually  changed  into  coal  by  the 


270      INTRODUCTION    TO    MODERN    CHEMISTRY. 


action  on  it  of  water  and  of  the  varying  external 
conditions  to  which  it  has  been  subjected  during  the 
millions  of  years  it  has  been  embedded  in  the  earth. 
Carbon  is  the  main  constituent  of  coal  (compare 
p.  212);  it  also  contains  hydrogen,  oxygen,  nitrogen, 
and  sulphur,  as  well  as  incombustible  matters  of 
different  kinds  which  are  grouped  to- 
gether under  the  common  name  of  ash. 


The    apparatus    sketched    in    fig.    52 


H 


Fig.  52. — Gas-making  apparatus  for  lecture  purposes. 

represents  the  essential  parts  of  a  manufactory  of  coal- 
gas.  Coal  has  been  placed  near  one  end  of  the  tube 
A,  and  this  tube  has  been  arranged  in  a  long  gas 
furnace  (B)  wherein  it  can  be  strongly  heated.  The 
open  end  of  the  tube  projects  from  the  furnace. 
This  end  carries  a  cork,  through  which  passes  a  glass 
tube  for  the  escape  of  any  volatile  bodies  that  may 
be  formed  when  the  coal  in  the  tube  is  heated. 

Let  us  consider  what  constituents  of  the  coal  can 
be  volatilised  from  it  by  this  treatment.  Let  us  begin 
with  the  hydrogen.  Part  of  the  hydrogen  of  the  coal 
will  immediately  escape  in  the  state  of  gas ;  another 


MANUFACTURE   OF   COAL-GAS.  2/1 

portion  will  be  volatilised  after  combining  with  carbon 
to  form  hydrocarbons.  We  know  that  the  number  of 
hydrocarbons  is  immense ;  hence  we  shall  not  be 
surprised  if  hydrocarbons  of  all  kinds  are  formed  at 
the  high  temperature  to  which  the  coal  is  raised  in 
the  retorts.  Moreover,  the  oxygen  in  the  coal  will 
react  with  the  carbon.  As  there  is  only  a  little  oxygen, 
the  main  product  of  this  reaction  will  be  carbon 
monoxide  gas :  a  little  carbon  dioxide  may  also  be 
produced.  As  regards  the  nitrogen,  part  of  it  will 
escape  as  nitrogen  gas,  but  part  will  combine  with 
hydrogen  to  form  ammonia,  which  will  pass  away  as 
a  gas.  Almost  all  of  the  sulphur  will  leave  the  retort  in 
combination  with  hydrogen,  as  sulphuretted  hydrogen 
gas.  The  main  constituent  of  coal  is  carbon ;  and  as 
only  a  small  part  of  this  will  be  removed,  in  combina- 
tion with  hydrogen  as  hydrocarbons  and  in  com- 
bination with  oxygen  as  carbon  monoxide,  the  greater 
part  of  the  carbon  will  remain  in  the  retorts,  as  carbon 
itself  is  quite  non-volatile.  When  the  coal  has  been 
heated  for  a  long  time  and  the  production  of  gas  has 
ceased,  the  carbon  which  has  not  been  volatilised  in 
combination  with  other  elements  will  be  found  in  the 
retorts,  along  with  the  ash  of  the  coal.  This  carbon  is 
known  as  coke.  As  coke  consists  almost  wholly  of 
carbon,  it  may  be  burnt  in  furnaces  and  stoves ;  for 
carbon  will  burn  when  it  is  ignited  and  sufficient  air 
is  supplied ;  and  it  is  the  lack  of  the  necessary  air 
which  makes  impossible  the  burning  of  the  coke  in  the 
retorts. 

How  will  the   volatile  products  behave   when  they 


2/2      INTRODUCTION    TO   MODERN    CHEMISTRY. 

leave  the  retorts  ?  Although  these  bodies  were  all 
gaseous  at  the  temperature  of  the  retorts,  and  escaped 
as  gases  from  the  retorts,  they  will  not  all  remain  in 
the  gaseous  state  at  the  ordinary  temperature.  Several 
of  these  substances  will  condense  to  liquids  in  the 
cooler  parts  of  the  apparatus  almost  as  soon  as  they 
leave  the  retorts.  This  part  of  the  products  of 
distillation  will  collect,  in  the  form  of  tar,  in  the  vessel 
c  (fig.  52).  In  a  manufactory  of  gas,  the  volatile 
substances  coming  from  the  retorts  are  very  thoroughly 
cooled,  for  the  purpose  of  removing  all  the  tar,  as  if 
any  of  that  were  left  it  would  tend  to  stop  the 
street-mains.  Everything  that  is  not  very  volatile  is 
thus  removed  from  the  gaseous  products  of  the  dis- 
tillation. The  stream  of  gas  is  now  passed  through 
a  tower  containing  coke  (compare  p.  69),  on  to  which 
a  stream  of  water  trickles  from  above.  (This  tower  is 
not  shown  in  fig.  52).  The  water  is  thus  made  to 
present  a  very  large  surface  to  the  gas.  We  know 
(see  p.  1 68)  that  ammonia  gas  is  extraordinarily 
soluble  in  water,  hence  we  are  not  surprised  that  all 
the  ammonia  in  the  coal-gas  should  be  removed  by  this 
process  of  washing.  Because  of  this  washing  whereto 
the  gas  must  be  submitted,  the  gas-works  become  the 
sources  of  the  ammonia  water  from  which  ammonia 
compounds  are  prepared  for  use  as  artificial  manures 
(compare  p.  215). 

The  gas  which  has  been  freed  from  ammonia  now 
passes  over  layers  of  hydrated  oxide  of  iron.  One  of 
our  first  experiments  (p.  11)  showed  how  readily  iron 
and  sulphur  combine.  In  the  present  case  the  sul- 
phuretted hydrogen  in  the  crude  gas  reacts  with  the 


MANUFACTURE   OF   COAL-GAS.  2/3 

hydrated  oxide  of  iron  and  forms  sulphide  of  iron ;  in 
this  way  the  gas  is  freed  from  sulphur,  in  the  flask  D 
(fig.  52).  If  sulphur  were  left  in  the  gas,  it  would  be 
burnt  to  sulphur  dioxide,  SO2  (compare  p.  140),  and 
the  smell  of  this  compound  would  make  it  impossible 
to  remain  in  a  room  lighted  by  such  gas. 

The  purification  of  the  gas  is  now  completed.  The 
purified  gas  collects  in  the  gasholder  (E,  fig.  52), 
wherein  it  is  stored  and  from  which  it  is  distributed 
to  the  consumers.  To  test  the  gas  we  have  made,  we 
ignite  a  jet  of  it,  after  opening  the  stopcock  H,  and 
we  see  that  it  burns  with  the  customary,  clear,  luminous 
flame  of  coal-gas. 

An  analysis  of  coal-gas,  prepared  as  has  been 
described,  gave  the  following  results  : — 

Hydrogen  (H)  .         .        .         .     45*2  volumes  per  cent. 
Methane  (CH4) .         .        .         .     35-0 
Other  hydrocarbons  ...       4-4         ,,  „ 

Carbon  monoxide  (CO)     .         .       8-6        „  „ 

Carbon  dioxide  (CO2)        .         .2-0        ,,  „ 

Nitrogen  (N)     .         ...       4-8        „ 

lOO'O 

The  most  important  of  the  compounds  that  are  classed 
as  "other  hydrocarbons"  are  ethylene,  H2ClHCH2,  and 
acetylene,  HCEECH,  hydrocarbons  containing  doubly 
linked  and  trebly  linked  atoms  of  carbon. 

Considering  what  we  have  now  learned,  we  shall  not 
be  surprised  to  .find  that  very  many  hydrocarbons  are 
obtained  from  tar.  By  distilling  tar,  in  an  apparatus 
like  that  represented  on  p.  7,  benzene  is  obtained — the 

18 


274      INTRODUCTION   TO    MODERN   CHEMISTRY. 

hydrocarbon  having  the  composition  C6H6  and  boiling 
at  80°  C.  [176°  F.],  which  we  have  already  considered 
so  fully.  Among  the  other  hydrocarbons  obtained 
from  tar  may  be  mentioned  naphthalene.  This  com- 
pound, which  has  the  formula  C10H8,  solidifies  on 
cooling.  Some  of  the  oxygen  in  the  coal  reacts  with 
constituents  of  the  tar,  and  (besides  benzene)  there  is 
formed,  among  other  bodies,  the  compound  C6H6O, 
or  C6H5 — OH.  This  compound  is  none  other  than 
carbolic  acid  (see  p.  254).  We  see,  then,  how  easy  it  is 
to  obtain  carbolic  acid  from  tar  :  it  is  only  necessary  to 
distil  the  tar.  Ammonia  is  not  the  only  nitrogenous 
compound  obtained  by  distilling  coal.  At  the  high 
temperature  of  the  retorts  a  small  portion  of  the  nitro- 
gen in  the  coal  goes  to  produce  ring-formed  compounds, 
among  which  is  pyridine  (see  p.  256). 


ACETYLENE  GAS. 

Of  late  years  there  has  come  into  use  for  lighting 
purposes  a  gas  which  is  much  more  easily  prepared 
than  coal-gas.  Notwithstanding  this  apparent  ad- 
vantage, this  gas  has  not  been  a  serious  rival  to  coal- 
gas  ;  it  has  not  taken  the  place  of  coal-gas,  as  that 
has  been  replaced  by  the  electric  light,  or  as  coal-gas 
replaced  candles. 

It  is  almost  self-evident  that  many  chemical  com- 
pounds should  be  formed  at  high  temperatures  which 
cannot  be  produced  at  lower  temperatures.  For  the 
last  ten  years  or  so  it  has  been  possible  to  attain 
temperatures  in  manufacturing  processes  much  higher 


THE   ELECTRIC   FURNACE.  275 

than  those  that  could  be  reached  before  that  time.  In 
order  to  obtain  high  temperatures,  we  now  make  direct 
use  of  the  arrangement  which  is  employed  in  the 
electric  arc  light  for  raising  the  carbon  points  to  a  full 
white  heat.  In  almost  all  the  methods  of  heating 
formerly  used,  the  heat  was  conducted  to  the  contents 
of  a  vessel  through  the  walls  of  the  vessel,  and  only  a 
portion  of  the  heat  of  the  external  fire  reached  the 
substance  to  be  heated.  But  in  the  new  method  the 


Fig.  53.— Electric  furnace. 

carbon  points  can  be  placed  in  contact  with  the  sub- 
stance to  be  heated,  so  that  the  substance  shall  be 
exposed  to  the  full  heat  of  the  electric  arc.  The 
remarkable  results  obtained  by  the  use  of  the  electric 
furnace  depend  on  this  fundamental  difference  between 
the  new  method  and  all  the  older  methods  of  heating 
(see  fig.  53).  For  instance,  if  a  mixture  of  carbon  and 
lime  is  heated  in  the  electric  furnace,  the  two  substances 
readily  react,  a  result  which  can  hardly  be  obtained 
by  any  other  method. 

Lime  is  calcium  oxide,   CaO.     When  a  mixture  of 


276      INTRODUCTION   TO   MODERN   CHEMISTRY. 

carbon  and  lime  is  exposed  to  the  high  temperature  of 
the  electric  arc,  the  calcium  and  carbon  combine,  and 
the  oxygen,  which  was  held  by  the  calcium,  enters  into 
union  with  another  portion  of  the  carbon  to  form  carbon 
monoxide  gas,  which  passes  away.  The  product  of  this 
reaction  in  the  electric  furnace  is  a  compound  of  calcium 
and  carbon,  called  calcium  carbide,  and  having  the 
composition  CaC2.  Calcium  carbide  is  now  an  im- 
portant commercial  article.  This  compound,  which  is 
perfectly  stable  at  the  high  temperature  of  the  electric 
furnace,  is  very  sensitive  to  the  action  of  cold  water. 
As  soon  as  calcium  carbide  comes  into  contact  with 
cold  water,  a  reaction  occurs,  in  accordance  with  the 
following  equation  : — 

CaC2          +  H2O  =         C2H2        +         CaO. 
Calcium  carbide  +  water  =  acetylene  gas  +  calcium  oxide. 

To  obtain  acetylene  gas,  nothing  more  is  needed  than 
to  pour  water  on  to  calcium  carbide.  The  gas  may  be 
produced  very  conveniently  by  using  a  Kipp's  appa- 
ratus (see  pp.  36  and  199).  Pieces  of  calcium  carbide 
are  put  into  the  middle  bulb,  the  stoppers  are  placed  in 
the  openings,  and  water  is  poured  into  the  apparatus, 
the  stopcock  F  being  kept  closed.  As  soon  as  the 
stopcock  is  opened,  water  flows  into  the  bulb  containing 
the  calcium  carbide,  and  acetylene  gas  is  produced,  in 
accordance  with  the  equation  given  above. 

Acetylene  gas  burns  with  a  very  clear  flame,  which, 
however,  smokes  very  readily,  because  of  the  extremely 
large  amount  of  carbon  contained  in  the  gas.  It  is 
easy  to  calculate  the  quantity  of  carbon  in  acetylene. 


ACETYLENE   GAS.  2/7 

As  the  atomic  weights  of  carbon  and  hydrogen  are  12 
and  I  respectively,  the  molecular  weight  of  acetylene  is 


(2  x  12)  +  (2x1)  =  26. 

Twenty-four  parts  by  weight  of  carbon  are  contained 
in  twenty-six  parts  of  acetylene.  Calculating  to  per- 
centage, we  have  26  :  24  =  100  :  x  ;  hence  x  =  92-3. 
Acetylene  thus  contains  92-3  per  cent,  of  carbon  and 
77  per  cent,  of  hydrogen. 

The  most  annoying  thing  about  acetylene  gas  is  the 
presence  in  it  of  gaseous  impurities,  especially  phos- 
phoretted  hydrogen.  We  know  that  all  commercial 
kinds  of  carbon  contain  ash,  and  that  calcium  phos- 
phate is  one  of  the  constituents  of  such  ash  (see  p.  206). 
That  compound  reacts  with  carbon  at  the  high  tempera- 
ture of  the  electric  furnace,  forming  carbon  monoxide, 
which  passes  off,  and  calcium  phosphide  ;  and  when 
calcium  phosphide  comes  into  contact  with  water,  phos- 
phoretted  hydrogen  is  evolved.  We  have  already 
noticed  that  phosphoretted  hydrogen  gas  is  spontane- 
ously inflammable  (p.  198).  Although  the  minute 
quantity  of  phosphoretted  hydrogen  in  acetylene  may 
not  cause  the  gas  to  take  fire  spontaneously,  yet  when 
the  acetylene  is  ignited  for  the  purpose  of  giving  light, 
the  phosphoretted  hydrogen  burns  to  phosphoric  acid 
and  water.  The  phosphoric  acid  makes  itself  visible, 
in  a  room  lighted  by  acetylene,  in  the  form  of  a  white 
smoke,  which  causes  headaches  and  other  ailments. 
Acetylene  cannot  be  generally  used  until  a  suitable 
means  of  purifying  it  has  been  found.  If  the  gas 


278      INTRODUCTION   TO   MODERN   CHEMISTRY. 

is  passed  over  calcium  chloride,  the  phosphoretted 
hydrogen  is  retained  :  but  much  remains  to  be  done 
in  improving  the  methods  of  purification. 


PETROLEUM. 

Liquid  combustible  material  is  obtained  in  many 
places  by  forming  suitable  bore-holes  in  the  earth's 
surface.  The  natural  product  is  called  crude  petroleum  ; 
when  this  is  distilled,  a  liquid  is  obtained  which  is 
suitable  for  burning  in  lamps.  The  examination  of 
this  substance  shows  that  it  is  composed  of  a  great 
many  hydrocarbons  belonging  to  the  class  of  open 
chain  compounds  (see  p.  249).  As  petroleum  is  quite 
free  from  oxygen  and  sulphur,  the  metal  sodium,  which 
so  eagerly  combines  with  oxygen,  may  be  kept  in  it 
without  undergoing  change.  As  the  only  products  of 
the  combustion  of  petroleum  are  carbon  dioxide  and 
water,  the  burning  of  this  substance  does  not  vitiate 
the  surrounding  air.  No  satisfactory  explanation  has 
yet  been  given  of  the  formation  of  petroleum  in  the 
interior  of  the  earth. 


FLAME. 

We  have  always  used  gas-flames  for  heating  ap- 
paratus ;  but  the  flames  we  have  used,  unlike  the 
ordinary  gas-flames,  have  been  non-luminous,  and  have 
not  deposited  soot  on  the  surfaces  exposed  to  them. 

The  luminosity  of  ordinary  gas-flames  is  brought 
about  in  the  following  way.  So  high  a  temperature  is 


LUMINOSITY   OF  FLAMES.  279 

attained  by  the  burning  of  the  hydrocarbons  in  the 
gas  that  a  portion  of  these  hydrocarbons  is  decom- 
posed, with  the  separation  of  carbon,  and  the  formation 
of  hydrogen,  which  burns  more  rapidly  than  the  carbon. 
It  is  only  in  the  outer  edges  of  the  flame  that  the 
separated  carbon  finds  oxygen  enough  to  burn  it,  and 
it  is  not  till  it  reaches  the  outside  of  the  flame  that 
this  carbon  disappears  as  it  is  changed  into  carbonic 
acid  gas.  The  carbon  which  separates  in  the  interior 
of  the  flame  is  raised  to  a  full  red  heat,  and  (like 
all  glowing  bodies)  emits  light,  which  we  use  for 
the  purpose  of  illumination. 

The  readiness  of  the  hydrocarbons  to  undergo  this 
decomposition  increases  as  the  amount  of  carbon  in 
them  increases.  The  richer  a  hydrocarbon  is  in  carbon, 
the  more  easily  is  carbon  separated  from  it.  This  readi- 
ness reaches  its  maximum  in  acetylene.  The  carbon 
that  is  separated  in  a  hot  flame  cannot  burn  in  the 
interior  of  the  flame  for  want  of  oxygen,  but  must 
pass  towards  the  outer  edge,  where  it  may  be  burnt. 
During  this  brief  time  the  carbon  is  raised  to  the  high 
temperature  that  is  needed  to  make  it  luminous.  What 
has  been  said  of  the  flame  of  coal-gas  holds  good  of 
other  luminous  flames,  such  as  those  of  oil  lamps  and 
candles.  In  the  case  of  an  oil  lamp  or  a  candle,  the 
wick  sucks  up  liquid  or  melted  material,  which  is 
gasified  in  the  flame,  wherein  it  suffers  a  kind  of  dry 
distillation  (compare  p.  269),  and  then  burns  luminously. 
Every  lamp  and  every  candle  is  a  gas-works  in  itself, 
and  produces  the  gas  which  it  requires. 

To   show  that  a  luminous  gas-flame    contains  free 


280      INTRODUCTION    TO   MODERN   CHEMISTRY. 

carbon,  it  is  only  necessary  to  hold  a  porcelain  plate 
in  such  a  flame.  By  the  sudden  cooling  of  the  flame 
thus  effected,  and  also  by  the  opposition  which  the 
plate  presents  to  the  passing  outwards  of  the  particles 
of  carbon,  some  of  the  particles  that  are  floating  about 
are  prevented  from  reaching  the  outside  of  the  flame. 
These  particles  cannot,  therefore,  be  burnt ;  they  settle 
on  the  plate  in  the  form  of  soot,  which  is  very  finely 
divided  carbon. 

It  is  such  hydrocarbons  as  acetylene  and  ethylene 
which  cause  the  luminosity  of  an  ordinary  gas  flame.* 
The  greater  the  quantity  of  such  hydrocarbons  in 
coal-gas,  the  greater  will  be  the  luminosity  of  the 
gas,  until  at  last  there  is  a  danger  of  the  flame 
smoking ;  that  is  to  say,  so  many  particles  of  carbon 
may  be  separated  in  the  interior  of  the  flame  that  they 
do  not  find  sufficient  oxygen  for  their  complete  com- 
bustion even  when  they  have  travelled  to  the  outer 
edge  of  the  flame,  so  that  some  of  them  get  cooled 
below  their  temperature  of  ignition  and  float  about  as 
soot. 

If  coal-gas,  before  it  is  burned,  is  mixed  with  so 
much  'air  that  there  will  be  enough  oxygen  in  the 
flame,  when  the  gas  is  ignited,  to  burn  all  the  carbon 
to  carbon  dioxide  inside  the  flame,  there  will  be  no 
separation  of  carbon — -there  will  be  no  formation  of 
carbon  which,  being  heated  very  strongly,  would  make 
the  flame  luminous  ;  in  other  words,  the  flame  will  not 

*  In  the  analysis  of  coal-gas  (on  p.  273)  these  and  other 
hydrocarbons  are  grouped  together  under  the  heading  "other 
hydrocarbons." 


THE   BUNSEN    BURNER. 


28l 


emit  light,  nor  will  it  smoke.  Such  a  flame  will  be 
suitable  for  heating  all  sorts  of  vessels,  and  especially 
for  cooking,  as  it  will  not  blacken  the  utensils  that  are 
used. 

Bunsen  solved  this 
problem  soon  after  the 
introduction  of  light- 
ing by  gas,  and  the 
principle  he  employed 
is  the  foundation  of 
all  the  apparatus  that 
is  used  to-day  for 
cooking  by  means  of 
non  -  luminous  gas- 
flames.  The  method 
is  based  on  the  ar- 
rangements shown  in 
fig.  54,  which  repre- 
sent the  special  form 
of  apparatus  that 
is  used  in  chemical 
laboratories.  The  gas 
is  admitted  at  B,  and 
issues  from  a  fine 
opening  at  c.  The 
burner  is  mounted  on 
a  heavy  foot,  which  gives  it  the  necessary  stability. 
If  the  stream  of  gas  is  ignited  at  c,  it  burns  in  the 
usual  way,  that  is  to  say,  luminously.  But  the  gas 
is  not  ignited  at  c  :  a  long  metal  tube  (D)  is  placed 
over  the  opening  »c,  and  is  fastened  by  a  screw- 
thread  (shown  on  A  and  also  on  D).  The  peculiarity 


-  54. — The  Bunsen  burner  and  its 
several  parts. 


282      INTRODUCTION   TO   MODERN   CHEMISTRY. 

of  this  metal  tube  is  that  it  is  pierced  near  its  lower 
end  by  holes  (E).  If  the  tube  D  is  fixed  over  the 
burner  and  gas  is  led  in  at  B,  the  gas,  streaming 
upwards  through  D,  will  draw  air  in  by  the  holes  E  ; 
hence  a  mixture  of  gas  and  air  will  issue  at  the 
upper  end  of  the  tube,  that  is,  at  F.  When  the 
mixture  of  gas  and  air  is  ignited,  at  F,  there  will  be 
air,  and  therefore  oxygen,  in  the  interior  of  the  flame ; 
hence  the  minute  particles  of  carbon  that  are  produced 
in  an  ordinary  gas-flame  will  not  be  formed  in  this 
flame,  for  all  the  carbon  will  be  burnt  by  the  oxygen 
that  is  mixed  with  the  gas.  The  flame  cannot,  there- 
fore, become  luminous.  The  apparatus  H  is  an  outer 
case  which  can  be  pushed  over  D.  The  amount  of  air 
admitted  through  the  holes  E  can  be  regulated  by 
turning  this  case.  The  flame  produced  by  such  a 
burner  as  this — a  burner  we  have  made  use  of  re- 
peatedly— heats  vessels  without  dirtying  them,  because 
there  is  no  separation  of  carbon  in  the  flame. 

The  flame  of  such  a  burner  as  that  just  described 
is  much  hotter  than  an  ordinary  gas-flame,  because  of 
the  rapid  combustion  of  the  whole  of  the  carbon  in 
the  gas.  If  a  bundle  of  platinum  wire  is  held  in  this 
flame — platinum  is  quite  unchanged  at  this  tempera- 
ture— it  becomes  red  hot  and  emits  light.  The  modern 
incandescent  gas-light  exhibits  this  process  in  an 
extremely  complete  way.  This  arrangement  resembles 
a  Bunsen  lamp.  There  are  holes  (generally  four)  near 
the  place  where  the  gas  enters,  so  that  it  is  a  mixture 
of  gas  and  air  that  is  burnt,  and  the  flame  is  non- 
luminous  and  very  hot.  The  mantle  is  suspended  in 


THE   INCANDESCENT   GAS-LIGHT. 


283 


the  non-luminous  flame  (see  fig.  55).     The  mantle  is 

made  of  cotton   which    has    been    thoroughly  steeped 

in  solutions  of  nitrate  of  thorium  and  nitrate  of  cerium 

[these  are  salts    of  two  comparatively  rare   metals]  ; 

the  cotton  is  then  strongly  heated,  when  it  burns,  and 

the  nitrates  are  decomposed,  nitrogen  and  oxygen  are 

given    off,    and   a  mixture    of  oxides  of  thorium  and 

cerium  remains  in  the  form  of  a  mantle, 

which,    unfortunately,    is   so    perishable. 

If  the  mixture  of  oxides  contains  99  per 

cent,    thorium    oxide    and    I     per    cent. 

cerium  oxide  (and  this  is  easily  attained 

by    steeping    the    cotton    mantle    in     a 

properly  mixed    solution  of  the  two  ni- 

trates), it  emits  an  extremely  clear  white 

light  when   it  is  strongly  heated  by   the 

non-luminous  flame  obtained  by  making 

use   of    the     principle    of    the    Bunsen 

burner. 

A  temperature  higher  than  that  of  the 
Bunsen  burner  is  obtained  by  blowing 
air  into  a  gas  flame.  The  gas  blow-pipe 
is  represented  in  fig.  56.  Gas  enters  at  A, 
and  burns  with  a  luminous  flame  :  air  is  driven  in  by 
bellows,  at  B,  and  passes  along  a  tube  which  terminates 
in  a  point  inside  the  flame.  As  soon  as  air  is  driven 
into  the  flame,  the  luminosity  vanishes,  and  a  very 
high  temperature  is  attained,  whereat  glass,  for  in- 
stance, melts  easily.  Should  the  temperature  thus 
attained  not  be  high  enough  for  some  special  purpose, 
oxygen  gas  may  be  blown  into  the  flame  in  place  of 


284      INTRODUCTION   TO   MODERN   CHEMISTRY. 

air ;  *  but  it  is  not  often  necessary  to  do  this  in  the 
laboratory.  This  flame  is  so  hot  that  it  may  take 

the  place  of 
the  oxyhydrogen 
flame  (see  page 
137)  in  almost 
every  case ;  and 
it  has  the  ad- 
vantage of  using 
coal-gas,  whereas 
hydrogen  must 
be  especially  pre- 
pared if  the  oxy- 
hydrogen flame 
is  to  be  em- 
ployed. 

SILICON. 

Silicon  is  an 
element  that  is 
chemically  very 
like  carbon ;  com- 
pounds of  it  are 

Fig.  56.— Blow-pipe. 

found  very  widely 

distributed  on  the  earth.  The  oxygen  compound,  SiO2, 
called  silicon  dioxide  [or,  more  commonly,  silica],  corre- 
sponds with  carbon  dioxide.  As  the  latter  is  often 
named  carbonic  acid,  so  is  SiO2  called  silicic  acid, 
although  the  more  correct  name  is  silicic  anhydride. 
Ordinary  sand  is  [more  or  less  pure]  silica,  SiO2. 

*  Oxygen  may  be  bought  in  steel  tubes. 


SILICATES.  285 

This  oxide  shows  its  acidic  character  by  combining 
with  bases  to  form  salts  ;  for  instance,  it  combines  with 
lime  to  form  calcium  silicate,  with  alumina  to  form 
aluminium  silicate,  and  with  potash  to  form  potassium 
silicate  (see  p.  74).  Silicates  play  an  important  part  in 
nature  :  most  of  the  rocks  whereof  mountains  are  formed 
consist  of  mixtures  of  these  salts  ;  granite,  for  instance, 
is  a  mixture  of  three  minerals — felspar,  quartz,  and  mica. 
Felspar  is  composed  of  the  silicates  of  aluminium, 
calcium,  and  potassium ;  quartz  is  crystallised  silica  ; 
and  the  chief  constituents  of  mica  are  silicate  of 
aluminium  and  silicate  of  magnesium. 

Of  the  many  attempts  that  have  been  made  to  prepare  a  com- 
pound of  one  atom  of  silicon  with  one  atom  of  oxygen  (SiO, 
corresponding  with  CO),  none  has  yet  been  successful. 

Silicon  forms  a  compound  with  hydrogen,  SiH4. 
This  compound,  which  is  called  silicon  hydride,  corre- 
sponds with  methane,  CH4.  Silicon  hydride  is  a  gas ; 
it  is  prepared  by  the  reaction  of  hydrochloric  acid  with 
a  compound  of  silicon  and  magnesium  (magnesium 
silicide}.  The  preparation  is  similar  to  that  of  arsen- 
uretted  hydrogen  (p.  218).  In  place  of  zinc  silicide,  the 
more  easily  prepared  compound  of  silicon  and  magnesium 
is  used. 

SiMg2      +        4HC1        =          SiHt          -f      2MgCl2. 

Magnesium         hydrochloric   _   silicon  hydride         magnesium 

silicide  acid  gas  chloride. 

As2Zri3    +       6HC1         =       2AsH3       +  3ZnCl2. 

Zinc  hydrochloric    _       arsenic  zinc 

arsenide  acid  ~   hydride  gas         chloride. 

Silico -chloroform,   SiHCl3  (analogous   to  chloroform^ 


286      INTRODUCTION    TO   MODERN    CHEMISTRY. 

CHC13,  p.  233),  can  be  obtained  by  replacing  three 
atoms  of  hydrogen  in  silicon  hydride  by  chlorine. 

Silicon  atoms  have  not  the  property  of  combining 
together  in  long,  branching  chains  in  so  marked  a 
way  as  atoms  of  carbon  ;  hence  the  number  of  silicon 
compounds  is  much  smaller  than  the  number  of  carbon 
compounds,  notwithstanding  that  the  two  elements  are 
similar  and  that  the  atoms  of  both  are  tetravalent. 

This  shortcoming  in  the  behaviour  of  silicon,  if  such 
an  expression  may  be  used,  is  particularly  helpful  in 
making  clear  to  us  that  it  is  not  the  tetravalency  of 
carbon  alone  which  gives  to  that  body  its  peculiar 
position  among  the  elements.  This  special  position 
depends  much  more  on  the  capability  which  the  atoms 
of  carbon  possess  of  combining  with  one  another  to 
form  chains  that  ramify  in  various  directions. 

We  shall  now  pass  from  the  consideration  of  non- 
metallic  elements  to  consider  some  of  the  metals. 


THE   METALS. 

THE!  metals  may  be  divided  into  two  main  classes,  the 
heavy  metals  and  the  light  metals.  Such  metals  as 
iron,  lead,  silver,  etc.,  belong  to  the  first  class ;  and  the 
second  class  contains  those  which  have  a  specific  gravity 
less  than  5.  Most  of  the  lighter  metals  have  been  dis- 
covered in  the  present  century. 

The  terms  noble  and  base  are  sometimes  used  to 
designate  different  classes  of  metals.  The  noble  metals 
are  found  in  the  earth  uncombined  with  other  elements. 
Although  they  have  been  in  contact  with  the  air  for 
endless  ages,  they  have  not  combined  with  the  oxygen 
of  the  air — they  have  not  changed  into  oxides.  Gold, 
platinum,  and  considerable  quantities  of  silver  are  found 
"  native."  The  base  metals,  on  the  other  hand,  are 
generally  found  combined  with  oxygen  or  sulphur. 
In  the  older  periods  of  the  earth's  history  the  metals 
must  have  had  many  opportunities  of  forming  com- 
pounds with  sulphur,  whose  chemical  likeness  to  oxygen 
we  already  know  (see  p.  267).  Those  sulphides  of 
metals  which  are  found  in  the  earth  are  spoken  of  as 
pyrites,  glance,  or  blende;  for  instance,  there  is  copper 
pyrites,  lead  glance,  and  zinc  blende.  Many  other  com- 
pounds of  the  base  metals  occur  more  complex  than 

287 


288      INTRODUCTION    TO   MODERN    CHEMISTRY 

the  oxides  and  sulphides — for  instance,   carbonate  of 
lead  and  sulphate  of  magnesium. 

When  a  metal  or  a  compound  of  a  metal  is  found 
in  the  earth  in  such  quantities  that  the  preparation  of 
the  metal  from  the  raw  material  is  technically  re- 
munerative, the  substance  found  in  the  earth  is  spoken 
of  as  an  ore  of  the  metal.  Whether  any  material  is  or 
is  not  considered  an  ore  of  a  certain  metal  depends 
entirely  on  the  commercial  value  of  the  material.  For 
instance,  the  dust  of  the  streets  always  contains  iron  ; 
no  one,  however,  would  think  of  extracting  iron  there- 
from ;  but  if  the  street  dust  contained  as  much  gold  as 
it  contains  iron,  it  would  be  a  first-rate  gold  ore. 

Metallurgy  is  concerned  with  the  extraction  of  metals 
from  their  ores.  In  the  case  of  a  noble  metal,  the 
operation  is  confined,  for  the  most  part,  to  the  mechanical 
separation  of  the  metal  from  the  soil  or  rock  wherein  it 
is  embedded.  All  the  platinum  and  a  good  deal  of  the 
gold  that  is  used  is  thus  obtained.  Although  some 
silver  is  got  in  this  way,  it  is  generally  necessary  to 
use  more  complicated  methods  for  obtaining  that  metal, 
as  most  of  it  is  found  in  combination  with  other  elements. 
In  the  case  of  a  base  metal,  it  is  always  necessary  to 
use  a  more  or  less  complex  process.  The  processes 
used  consist,  in  broad  outline,  in  so  treating  the  ores, 
however  complicated  they  may  be,  that  the  metals  in 
them  are  finally  converted  into  oxides.  The  oxides 
are  then  mixed  with  carbon  and  strongly  heated.  The 
carbon  seizes  the  oxygen  and  is  converted  into  carbon 
monoxide  gas,  and  the  metals  are  obtained  freed  from 


!  PREPARATION   OF   METALS.  289 

oxygen.  In  this  process  the  carbon  is  said  to  reduce 
the  metallic  oxide  to  metal.  The  following  equations 
present  instances  of  such  processes  of  reduction  : — 

CuO        +      C       =     Cu      +  CO. 

Copper  oxide    +   carbon  =  copper   +   carbon  monoxide  gas. 

NiO        +        C      =      Ni      +  CO. 

Nickel  oxide  +   carbon  =  nickel     +   carbon  monoxide  gas. 

Fe2O3          +     3C       =     2Fe    +  3CO. 

Ferric  oxide    +    carbon   =     iron      +    carbon  monoxide  gas. 

We  notice  that  the  carbon  which  reduces  the 
metallic  oxides  is  always  oxidised  to  carbon  monoxide 
(CO),  never  to  carbon  dioxide  (COa).  At  the  high 
temperatures  of  the  furnaces  wherein  the  operations 
are  conducted  the  carbon  cannot  bind  to  itself  more 
than  a  single  atom  of  oxygen. 

There  is  no  process  for  obtaining  the  metals  directly 
from  their  sulphur  compounds,  corresponding  with  that 
whereby  they  are  obtained  from  their  oxides,  which  is 
applicable  in  metallurgy.  An  indirect  method  must 
be  employed.  A  method  often  used  is  known  as 
roasting  the  ore.  The  sulphide  is  strongly  heated  in 
a  stream  of  air ;  the  oxygen  of  the  air  burns  the  sulphur 
to  sulphur  dioxide  gas  (SO2),  which  escapes  (see  p.  149), 
and  the  metal  is  converted  into  oxide,  which  remains : 
the  oxide  is  then  reduced  by  heating  with  carbon.  The 
roasting  of  zinc  blende  (which  is  the  source  of  most  of 
the  zinc  used  for  various  purposes)  and  its  conversion 
into  zinc  oxide  is  an  example  of  this  process. 

ZnS         +       3O       =        ZnO      +  SO2. 

Zinc  blende    +     oxygen    =    zinc  oxide  +  sulphur  dioxide  gas. 
(zinc  sulphide)      (from  the  air) 

19 


290      INTRODUCTION    TO   MODERN   CHEMISTRY. 

The  reduction  of  metallic  oxides  by  heating  them 
with  carbon  is  sometimes  conducted  in  a  blast  furnace. 
The  blast  furnace  is  a  tubular  erection,  into  which 
ore  and  carbon  [in  the  form  of  coal,  coke,  or  charcoal] 
are  thrown  from  above,  along  with  certain  substances 
which  serve  as  fluxes  and  are  necessary  for  properly 
conducting  the  smelting  process,  while  a  blast  of  air 
is  forced  in  from  beneath,  whereby  the  carbon  is  burnt 
in  the  furnace.  The  melted  metal,  and  the  other 
molten  substances  known  as  slag,  are  drawn  out  from 
below,  from  time  to  time,  while  fresh  material  is  thrown 
in  from  above. 

Iron  is  one  of  the  most  remarkable  of  the  heavy 
metals,  inasmuch  as  it  can  be  obtained  in  three  very 
different  modifications,  which  seem  to  be  three  different 
metals — namely,  pig-iron,  steel,  and  malleable  iron.  The 
chemical  differences  between  these  are  dependent  on 
the  different  quantities  of  carbon  they  contain.  All 
iron  that  is  technically  useful  must  contain  a  definite 
quantity  of  carbon ;  chemically  pure  iron  is  too  soft  to 
be  of  any  technical  use.  The  quantities  of  carbon  in 
the  three  kinds  of  iron  are  as  follows : — 

Pig-iron  contains   2*3,  or  more  than   2*3,  per  cent. 

of  carbon. 
Steel  contains   r6,  or   less  than    r6,  per  cent,   of 

carbon. 
Malleable  iron  contains  not  more  than  0*5  per  cent. 

of  carbon. 

Iron  containing  between   r6  and    2-3    per  cent,    of 
carbon  finds  no  technical  applications. 

Pig-iron  can  be  melted,  and  can,  therefore,  be  cast 


PREPARATION   OF   METALS.  291 

in  moulds.  Malleable  iron  and  steel  can  be  forged  ; 
that  is  to  say,  these  substances  do  not  become  fluid  at 
a  full  red  heat,  but  they  become  so  soft  that  they  can 
be  worked  with  the  hammer,  or  some  other  form  of 
pressure,  and  brought  into  any  shape  that  is  desired. 
Articles  of  steel  become  extraordinarily  hard  when  they 
are  cooled  suddenly  after  having  been  worked  under 
the  hammer,  but  malleable  iron  does  not  harden  when 
treated  in  this  way.  Steel  and  malleable  iron  can  be 
welded ;  that  is  to  say,  although  they  do  not  melt  at  a 
very  high  temperature,  two  pieces  can  be  united  to 
form  one  homogeneous  substance  by  hammering  them 
together  at  a  white  heat.* 

All  the  heavy  metals  commonly  used,  except  mercury 
and  zinc,  are  obtained  by  reducing  their  oxides  by 
heating  with  carbon  in  furnaces.  It  is  well  known  that 
mercury  volatilises  comparatively  easily.  If  an  attempt 
were  made  to  obtain  this  metal  by  heating  in  a  blast 
furnace,  the  metal  would  simply  escape,  as  a  gas,  into 
the  air  at  the  top  of  the  furnace.  Mercury  must  be 
distilled  from  retorts,  like  any  other  liquid,  like  water, 
for  instance.  The  retorts  used  for  distilling  mercury 
are  made  of  fire-clay.  The  metal  is  obtained  by  heating 
its  ores  in  such  retorts,  and  cooling  the  vapour  in 
suitable  vessels.  Zinc  also  must  be  got  by  a  similar 
process  ;  for  zinc  is  a  comparatively  easily  volatilised 
metal,  although  one  is  not  accustomed  to  think  of  it  as 
such.  If  zinc  oxide  is  heated  with  carbon,  the  metal  is 

*  More  details  concerning  the  domestic  and  economic  appli- 
cations of  iron  will  be  found  in  the  author's  Chemistry  in  Daily 
Life,  pp.  274-302  (2nd  Ed.) 


292      INTRODUCTION    TO   MODERN    CHEMISTRY. 

produced;  but  it  volatilises  at  the  high  temperature 
which  is  required  for  the  reduction  of  the  oxide  :  hence, 
if  the  process  were  conducted  in  a  blast  furnace,  the 
zinc  would  escape,  as  gas,  from  the  top  of  the  furnace, 
and  would  there  be  burnt  to  oxide  by  the  oxygen  in  the 
air.  For  these  reasons  zinc  oxide 
is  heated  with  carbon  in  fire-clay 
retorts,  and  the  vapour  of  zinc 
is  conducted  into  suitable  vessels, 
wherein  it  condenses.  Under  these 
conditions  the  vapour  of  the  metal 
come  into  contact  with 
has  no  chance  of  be- 
again  to  oxide. 

The  following 
experiment  shows 
how  readily  zinc 
may  be  volatilised, 
and  how  easily  its 
vapour  may  be 
burnt  in  the  air. 
A  few  small  pieces 
of  zinc-foil  are 

PlaCed  ln  * 


Fig.  57-Burning  zinc  by  a  blowpipe  flame. 

crucible,     which    is 

then  heated  by  the  flame  of  a  blow-pipe  (see  fig.  57). 
The  zinc  soon  melts,  and,  as  the  heating  is  continued, 
a  flame  appears  at  the  mouth  of  the  crucible,  and  zinc 
oxide  rises  into  the  air  as  a  white  smoke. 

Alloys  are  formed    by  heating  mixtures  of  metals. 


THE  OXIDES  OF   METALS.  293 

The  alloys  that  are  formed  by  dissolving  metals  in 
mercury  (and  mercury  dissolves  most  of  the  metals) 
are  called  amalgams.  Bronze  is  obtained  by  melting 
together  copper  and  tin,  brass  by  melting  together 
copper  and  zinc.  Gold  and  silver  are  soft  metals — so 
soft  that  they  are  worn  away  by  much  use ;  but  if 
mixtures  of  about  ninety  parts  of  gold  or  silver  with 
about  ten  parts  of  copper  are  melted,  the  very  hard 
alloys  are  obtained  of  which  our  gold  and  silver  coins 
are  made.* 

We  know  (compare  p.*2i7)  that  bases  are  oxides  of 
metals  :  these  bases  react  with  acids,  which  are  derived 
from  oxides  of  non-metals,  to  form  salts.  Many  metals, 
like  some  of  the  non-metals  we  have  considered,  com- 
bine with  oxygen  in  more  than  one  proportion  (we 
recall  the  two  oxides  of  sulpur,  SO2  and  SO3).  Iron, 
for  instance,  forms  the  oxide  FeO,  called  ferrous  oxide, 
and  the  oxide  Fe2O3,  called  ferric  oxide.  Halogen  com- 
pounds of  the  metals  generally  exist  corresponding 
with  such  oxides  ;  these  are  designated  -ous  and  -ic 
chlorides,  bromides,  and  iodides.  Ferrous  chloride 
has  the  composition  FeCl2,  and  ferric  chloride  has  the 
composition  FeCl3.  As  the  valency  of  an  elementary 
atom  is  measured  by  the  number  of  monovalent  atoms 
it  can  hold  fast  (compare  p.  229),  we  see  that  in  one  of 

*  The  English  gold  coinage  alloy  contains  gif  per  cent,  of 
gold  and  8^  per  cent,  of  copper  ;  the  silver  coinage  alloy  contains 
92^  per  cent,  of  silver  and  7^  per  cent,  of  copper.  The  German 
gold  coins  are  made  of  an  alloy  of  90  per  cent,  gold  and  10  per 
cent,  copper,  and  German  silver  coins  of  an  alloy  of  90  per  cent, 
silver  and  10  per  cent,  copper.  [TR.]j 


294      INTRODUCTION   TO   MODERN    CHEMISTRY. 

these  chlorides  the  iron  atom  is  divalent,  while  in  the 
other  it  is  trivalent.  Some  metals  exhibit  varying 
valency.  There  is  no  abrupt  distinction  between  the 
metals  and  the  non-metals  in  this  respect.  Nitrogen 
and  phosphorus  generally  act  as  trivalent  atoms,  but 
in  some  compounds  the  atoms  of  these  elements  are 
pentavalent ;  for  instance,  when  ammonium  chloride, 
NH4C1,  is  formed  from  ammonia,  NH3,  and  hydro- 
chloric acid,  HC1,  the  nitrogen  atom  must  hold  together 

H\  /H 

the   five   monovalent   atoms,    thus         >N<~H.      The 

CK       \  H 
atom  of  phosphorus  is  pentavalent  in  the  molecule  of 

Civ.  Cl 

phosphorus    pentachloride,         YP^-CL        But    while 

CK        \C1 

nitrogen  and  phosphorus  are  pentavalent  only  in  a 
proportionately  small  number  of  compounds,  many 
metals  show  one  valency  in  one  series  of  compounds 
and  another  valency  in  another  series  of  compounds, 
so  that  one  may  say  that  changing  valency  is  the  rule 
with  them,  while  it  is  something  exceptional  with 
nitrogen  and  phosphorus. 

Manganese,  which  is  a  metal  resembling  iron,  is  an 
especially  instructive  example  of  changing  valency. 
In  the  oxide  MnO,  called  manganous  oxide,  the  atom 
of  manganese  is  divalent,  inasmuch  as  oxygen  is  di- 
valent. This  oxide  forms  salts — for  instance,  MnSO4 
— by  reacting  with  acids. 

MnO  +        H2$04        =  MnS04  +    H2O. 

Manganous  oxide  +  sulphuric  acid  =  manganous  sulphate  +  water. 


COMPOUNDS  OF   MANGANESE.  295 

Manganese  also  forms  the  oxide  Mn2O3,  manganic 
oxide.  As  two  atoms  of  the  metal  are  here  combined 
with  three  atoms  of  divalent  oxygen,  each  atom  of 
manganese  in  this  compound  is  trivalent.  As  two 
atoms  of  manganese  take  the  place  of  six  atoms  of 
hydrogen,  the  sulphate  corresponding  with  this  oxide 
will  be  Mn2(SO4)3. 

Mn203          +       3H2S04         =         Mn2(SO4)8         +  3H2O. 
Manganic  oxide    +   sulphuric  acid    =   manganic  sulphate   +   water. 

The  compounds  derived  from  manganows  oxide  are 
called  manganows  compounds  ;  those  derived  from  man- 
gan/£  oxide  are  called  mangantc  compounds.  Similarly, 
FeSO4  is  ferrous  sulphate  (FeO  is  ferrous  oxide),  and 
Fe2(SO4)3  is  ferric  sulphate  (Fe2O3  is  fernic  oxide). 
The  equations  which  present  the  formations  of  these 
two  sulphates  of  iron  from  their  corresponding  oxides 
are  as  follows  :— 

FeO         4-         H2SO4       =          FeSO4          +    H2O. 
Ferrous  oxide  +  sulphuric  acid   —   ferrous  sulphate   +   water. 

Fe203      +         3H2S04       =       Fe2(S04)3        +  3H2O. 
Ferric  oxide    +   sulphuric  acid  =    ferric  sulphate     +   water. 

Besides  ferrous  and  ferric  oxides,  an  oxide  of  iron 
intermediate  between  these  is  known ;  it  is  called 
ferroso-ferric  oxide,  and  has  the  composition  Fe3O4 
[FeO .  Fe2O3].  This  oxide  is  found  abundantly  in 
nature ;  its  mineralogical  name  is  magnetic  ironstone. 
There  are  no  salts  corresponding  with  this  oxide. 

Returning  to  manganese,  mention  must  be  made  of 
the  oxide  MnO2,  called  manganese  peroxide  :  here  the 


296      INTRODUCTION   TO   MODERN   CHEMISTRY. 

manganese  atom  is  tetravalent.  No  salts  corresponding 
with  this  oxide  have  been  isolated.  We  are  now  in  a 
position  to  examine  more  fully  the  reaction  between 
this  oxide  and  hydrochloric  acid  which  we  used  for  the 
preparation  of  chlorine.  The  two  atoms  of  oxygen 
in  MnO2  are  able  to  oxidise  four  atoms  of  hydrogen 
to  water  ;  one  molecule  of  MnO2  reacts,  therefore,  with 
four  molecules  of  hydrochloric  acid,  HC1.  It  is  probable 
that  the  first  product  of  the  reaction  is  a  tetrachloride, 
MnCl4  (and  water).  If  this  salt  is  formed,  it  is 
very  unstable,  and  at  once  decomposes  to  manganous 
chloride,  MnG2  (corresponding  with  manganous  oxide, 
MnO),  and  chlorine.  The  equation  which  expresses 
the  preparation  of  chlorine  is: — 


+         4HC1        =       C12       +      MnCl2      +  2H2O. 
Manganese         hydrochloric  _   chlorine         manganous 
peroxide  acid  gas  chloride 

There  are  two  other  highly  oxidised  compounds  of 
manganese  —  manganic  acid,  wherein  the  manganese 
atom  seems  to  be  hexavalent,  and  permanganic  acid. 
Manganic  acid  has  not  itself  been  isolated,  but  its 
composition  may  be  deduced  from  that  of  its  potassium 
salt,  K2MnO4,  the  expanded  formula  of  which  may  be 

written  thus: 


The  anhydride  of  permanganic  acid,  Mn2O7,  has  been 
isolated,  by  adding  sulphuric  acid  to  potassium  per- 
manganate ;  it  is  called  also  manganese  heptoxide.  The 
salt  potassium  permanganate  is  often  used  [as  a  dis- 
infectant and  deodoriser;  a  solution  of  it  is  known 
as  "Condy's  fluid"].  The  formula  of  this  salt  is 
K2Mn2O8  (but  the  half-formula  KMnO4  is  often  used)  ; 


POTASSIUM   PERMANGANATE.  297 

and  the  composition  of  permanganic  acid,  from  which 
it  is  derived,   is  given   by  the  formula  H2Mn2O8. 

The  technical  productions  of  potassium  manganate 
and  potassium  permanganate  are  closely  connected. 
When  manganese  peroxide,  MnO2,  is  fused  with  potash, 
KOH,  and  potassium  chlorate,  KC1O3  (the  mineral 
pyrolusite  is  used ;  it  is  more  or  less  pure  MnO2),  the 
oxygen  of  the  chlorate  oxidises  the  MnO2,  and  the  pro- 
duct of  the  reaction — a  green  solid — contains  the  salt 
K2MnO4,  potassium  manganate.  If  this  is  dissolved 
in  water  (it  dissolves  easily)  and  chlorine  is  passed 
into  the  solution,  one  atom  of  the  potassium  of  the 
manganate  combines  with  chlorine,  forming  potassium 
chloride,  KC1,  and  the  solution  becomes  pinkish  red, 
because  of  the  formation  of  potassium  permanganate. 

2K2MnO4  +      C12      =      K.,Mn2O8      +       2KC1. 

potassium  potassium 

Potassium  manganate  +  chlorine  =  permanganate  +      chloride 

As  potassium  permanganate  is  less  soluble  in  water 
than  potassium  chloride,  the  two  salts  may  be  separated 
by  crystallisation.  After  evaporation  the  permanganate 
crystallises  out,  while  the  chloride  remains  in  the 
mother-liquor.  The  addition  of  an  acid  to  a  (green) 
solution  of  potassium  manganate  withdraws  potassium 
[forming  a  potassium  salt  of  the  acid  used],  and  pro- 
duces a  (red)  solution  of  potassium  permanganate. 
Manganic  acid  cannot,  then,  be  obtained  by  the  reaction 
of  potassium  manganate  with  an  acid. 

We  shall  not  attempt  to  treat  of  the  remaining  heavy 
metals ;  we  shall  content  ourselves  with  giving  the 


298      INTRODUCTION    TO   MODERN   CHEMISTRY. 

formulae  of  the  oxides  of  the  better  known  of  them. 
If  the  formulae  of  the  oxides,  and  hence  the  valencies 
of  the  elements,  are  known,  it  is  easy  to  deduce  the 
formulae  of  the  other  compounds  and  salts  of  these 
elements. 

Zinc,  Zn,  reacts  only  as  a  divalent  element ;  the  formula 
of  its  oxide  is  ZnO.  Two  oxides  of  mercury  are  known — 
Hg2O,  mercurous  oxide,  and  HgO,  mercuric  oxide ; 
the  former  yields  the  mercurous  salts,  and  the  latter 
the  mercuric  salts.  Copper  forms  cuprous  oxide,  Cu2O, 
and  cupric  oxide,  CuO  :  cuprous  and  cupric  salts  are 
known.  The  oxides  of  gold  are  aurous  oxide,  Au2O, 
and  auric  oxide,  Au2O3,  and  from  these  aurous  and 
auric  salts  are  derived.  The  only  oxide  of  silver  which 
forms  salts  is  Ag2O  :  there  is  another  oxide  known — 
silver  peroxide,  AgO — but  salts  corresponding  with  this 
oxide  have  not  been  isolated.  The  oxide  of  tin,  SnO2, 
reacts  with  strong  acids  as  a  base,  but  with  strong 
bases  it  acts  as  an  acid  ;  it  is  called  stannic  oxide, 
or  stannic  anhydride.  Lead  forms  three  oxides  :  PbO, 
(known  as  litharge),  which  reacts  with  acids  to  form 
salts ;  PbO2,  called  lead  peroxide  [corresponding  with 
which  only  one  salt,  lead  tetracetate,  Pb(C2H3O2)4,  has 
been  isolated]  ;  and  Pb3O4,  known  as  red  lead.  Of  the 
three  oxides  of  cobalt,  CoO,  Co2O3,  and  Co3O4,  only  the 
first  and  second  form  salts.  Nickel  forms  nickelous 
oxide,  NiO,  which  is  salt-forming,  and  nickelic  oxide, 

NiA- 

THE  LIGHT  METALS. 

While  many  of  the  heavy  metals  have  been  known 
for  a  very  long  time,  most  of  the  light  metals,  such  as 


THE   ALKALIS.  299 

potassium,  sodium,  and  aluminium  (which  have  been 
referred  to  on  p.  287),  were  prepared  for  the  first  time  in 
the  nineteenth  century.  The -alkalis  and  the  soluble 
bases,  those  exact  opposites  of  the  acids,  are  com- 
pounds of  certain  of  the  light  metals ;  potash,  for 
instance,  which  has  been  used  as  an  alkali  for  cen- 
turies, is  a  compound  of  the  metal  potassium.  The 
oxide  of  potassium  has  the  formula  K2O  ;  the  atom 
of  potassium  is  monovalent,  and  that  of  oxygen  is 
divalent.  This  oxide  reacts  with  water  to  form  the 
alkali  caustic  potash,  according  to  the  equation  : — 

K2O  +  H2O  =        2KOH. 

Potassium  oxide  +  water  =  caustic  potash. 

An  aqueous  solution  of  a  caustic  alkali  may  be  used 
as  the  exact  reverse  of  an  aqueous  solution  of  an  acid, 
say,  sulphuric  acid.  The  solution  of  such  an  acid  is 
neutralised  by  adding  to  it  a  solution  of  a  caustic  alkali. 
A  piece  of  paper  soaked  in  litmus  solution  and  dipped 
into  the  liquid  serves  to  indicate  when  sufficient  alkali 
has  been  added,  for  a  neutral  liquid  does  not  alter  the 
colour  of  either  red  or  blue  litmus.  If  carbonic 
anhydride  gas  or  sulphur  dioxide  gas  is  passed  into  a 
solution  of  caustic  potash,  potassium  carbonate  or 
potassium  sulphate  is  soon  formed. 

The  oxides  of  the  heavy  metals  also  react  with  the 
acids  to  form  salts  :  iron  oxide  and  sulphuric  acid  form 
sulphate  of  iron.  But  most  of  the  oxides  of  the  heavy 
metals  (ferric  oxide,  for  instance)  are  not  soluble  in 
water,  whereas  the  oxides  of  potassium  and  sodium 
dissolve  very  easily  in  water,  forming  caustic  potash 
and  caustic  soda  solutions  respectively.  Hence,  because 


300      INTRODUCTION   TO   MODERN   CHEMISTRY. 

of  their  insolubility  in  water,  it  is  not  nearly  such  an 
easy  and  rapid  process  to  neutralise  acids  by  the  oxides 
of  the  heavy  metals  as  to  neutralise  them  by  solutions 
of  caustic  potash,  soda,  or  ammonia. 

Very  many  compounds  of  certain  light  metals  have 
been  known  for  long.  Oxide  of  calcium,  CaO,  for 
instance,  is  obtained  by  strongly  heating  limestone 
(calcium  carbonate,  CaCO3),  which  is  a  very  widely 
distributed  and  common  substance.  When  this  com- 
pound is  strongly  heated,  it  decomposes  into  calcium 
oxide  and  carbon  dioxide  gas  : — 

CaCO3  CaO          +          CO2. 

Calcium  carbonate   =   calcium  oxide    +    carbon  dioxide, 
(limestone)  (burnt  lime) 

Burnt  lime  has  long  been  known  and  used  in  making 
mortar;  but  neither  the  alchemists  nor  the  early 
chemists  were  able  to  separate  it  into  oxygen  and  a 
metal.  Chemists  were  thus  in  the  awkward  position 
of  having  to  deal  with  a  substance  which  did  not  give 
one  the  impression  of  being  an  element — for  burnt  lime 
certainly  does  not  seem  to  be  an  elementary  substance 
— but  yet  mocked  every  attempt  to  separate  it  into 
simpler  constituents,  and  had,  therefore,  to  be  classed 
among  the  elements.  It  had  often  been  remarked  that 
burnt  lime  resembled  the  oxides  of  the  metals  ;  that, 
for  instance,  like  these  compounds,  it  neutralised  acids. 
Because  of  this  likeness,  it  was  customary,  before  the 
discovery  of  oxygen,  to  speak  of  what  we  now  call 
metallic  oxides  as  metallic  calces.  Lavoisier  (see  p.  1 27) 
was  the  first  to  prove  that  the  calx  of  tin  was  an  oxide 


ELECTROPLATING.  301 

of  tin.  We  can  perhaps  form  some  conception  of  the 
sensation  that  was  caused  by  the  isolation  of  the  first 
light  metal  by  Davy  in  the  year  1806.  The  metal  was 
potassium,  obtained  by  Davy  from  caustic  potash. 
The  isolation  of  that  metal  was  not  effected  by  purely 
chemical  methods,  but  by  the  help  of  electricity, 
obtained  by  using  the  "  Voltaic  pile,"  sixteen  years 
after  the  discovery  of  the  galvanic  current.  As  is  well 
known,  the  Voltaic  pile  has  long  been  superseded  by 
the  galvanic  cell,  and  that  is  being  now  replaced  by 
the  dynamo.  The  sensation  which  was  caused  by  the 
isolation  of  the  first  alkali  metal,  with  its  remarkable 
properties,  was  no  less  than  that  caused  in  our  own 
day  by  the  discovery  of  the  Rontgen  rays. 


PREPARATION  OF  THE  LIGHT   METALS    BY    ELECTRICITY. 

On  p.  33  we  learnt  that  the  electric  current  decom- 
poses water  into  oxygen  and  hydrogen.  If  we  use  an 
aqueous  solution  of  a  metallic  salt — say,  copper  sulphate 
— in  place  of  water,  the  current  causes  the  separation 
of  the  metal,  in  this  case  copper,  at  the  negative  pole 
(in  place  of  hydrogen),  and  the  radicle  SO4,  which  was 
combined  with  copper,  travels  to  the  positive  pole. 

This  process  is  applied  technically ;  it  is  called 
electroplating.  Suppose  it  is  wished  to  cover  a  piece 
of  zinc  with  nickel  (to  repeat  on  a  small  scale  a  process 
which  has  extended  greatly  of  late  years),  the  opera- 
tion is  carried  out  in  the  manner  shown  in  fig.  58. 
The  zinc  plate  A,  whereon  nickel  is  to  be  deposited, 
is  fastened  to  the  negative  pole  of  an  electric  circuit, 


302      INTRODUCTION   TO   MODERN   CHEMISTRY. 

and  a  plate  of  nickel  is  attached  to  the  positive  pole  B. 
Both  plates  are  immersed  in  a  solution  of  a  salt  of 
nickel,  say,  nickel  sulphate,  and  the  electric  current  is 
sent  through  this  nickel  bath.  After  a  short  time  we 
see  that  the  zinc  plate  is  covered  with  a  film  of  nickel. 

If  the  current  were  sent  through  a  solution  of  a  salt 
of  a  light  metal,  through  a  Solution  of  common  salt 
(sodium  chloride),  for  instance,  the  reaction  would,  of 


Fig.  58.— Electroplating. 

course,  be  similar.  Sodium  would  be  separated  at  the 
negative  pole.  But  we  know  (p.  29  )  that  sodium  in- 
stantly decomposes  water ;  therefore  it  is  not  possible 
that  sodium  should  be  actually  separated  in  this  experi- 
ment :  the  sodium  will  decompose  water,  and  hydrogen 
will  be  obtained  at  the  negative  pole.  If,  then,  it  is 
desired  to  prepare  one  of  the  alkali  metals  by  means 
of  the  electric  current,  not  a  trace  of  water  must  be 
present,  and  an  aqueous  solution  of  a  salt  of  the  alkali 
metal  must  not  be  used.  Now,  not  only  do  aqueous 


PROPERTIES   OF   POTASSIUM.  303 

solutions  of  the  salts  of  the  alkali  metals  conduct  elec- 
tricity, but  the  melted  salts  themselves  do  the  same. 
As  there  is  no  water  in  the  liquid  formed  by  melting 
one  of  these  salts,  the  alkali  metal  which  is  separated 
by  the  electric  current  remains  unchanged  therein,  and 
collects  at  the  negative  pole,  from  which  it  may  be 
removed.  This  was  the  method  employed  by  Davy. 


POTASSIUM. 

Potassium  hydroxide  (caustic  potash)  is  the  most 
convenient  compound  of  potassium  from  which  to 
obtain  the  metal  electrolytically.  The  compound  is 
melted,  and  the  current  is  sent  through  the  melted 
substance ;  potassium  collects  at  the  negative  pole. 

Potassium  is  a  silver-white  metal ;  its  specific  gravity 
is  0*86.  One  litre  of  this  metal  weighs  860  grams, 
while  one  litre  of  water  weighs  1000  grams.  The 
metal  therefore  floats  on  water.  It  decomposes 
water  into  its  constituents ;  the  hydrogen  is  given  off, 
and  the  metal  combines  with  the  oxygen,  forming 
potassium  oxide,  which  dissolves  in  the  rest  of  the 
water  with  the  production  of  potassium  hydroxide. 

Because  of  its  readiness  to  combine  with  oxygen, 
potassium  cannot  be  kept  exposed  to  the  air.  The 
surface  of  a  freshly  cut  piece  of  potassium  is  silver- 
white,  but  if  exposed  to  the  air  it  is  very  soon  covered 
with  a  greyish  film  of  oxide.  The  metal  is  much  softer 
than  lead ;  it  can  be  cut  by  a  knife  very  easily. 
Potassium  is  generally  kept  under  petroleum  (as 


304      INTRODUCTION   TO   MODERN    CHEMISTRY. 

sodium  is,  see  p.  29  ),  which  is  a  mixture  of  hydro- 
carbons (p.  278),  and  protects  the  potassium  from  the 
oxidising  action  of  the  air. 

We  have  already  made  mention  of  the  existence  of 
potassium  salts  in  the  ashes  of  plants,  and  also  of  the 
immense  deposits  of  these  salts  which  are  worked  at 
Stassfurt.  Until  the  Stassfurt  salts  began  to  be  worked, 
about  forty  years  ago,  the  ash  of  plants  was  the  only 
available  source  of  potassium  compounds.  When 
plants  or  wood  are  burnt,  the  carbon  forms  carbon 
dioxide  ;  and  this  combines  with  potassium,  if  com- 
pounds of  that  metal  are  present,  to  form  potassium 
carbonate.  The  formulae  of  carbon  dioxide  and  hypo- 
thetical carbonic  acid  are  these  : — 

CO2  C02  +  H2O  =  H2CO3 

Carbon  dioxide.  Hypothetical  carbonic  acid. 

Although  carbonic  acid  has  not  been  isolated  (com- 
pare p.  266),  we  must  regard  carbonates  as  derived 
from  this  compound.  If  we  suppose  that  the  two 
hydrogen  atoms  of  carbonic  acid  are  replaced  by 
potassium,  we  obtain  potassium  carbonate,  K2CO3. 
This  salt  used  to  be  called  pot-ashes,  for  the  following 
reasons.  The  lixiviation  of  the  ashes  of  wood,  for  the 
purpose  of  obtaining  a  substance  that  was  much  used 
in  making  soap  (p.  313),  glass  (p.  319),  and  in  other 
industries,  is  a  very  ancient  process ;  the  liquor  (con- 
taining potassium  carbonate)  was  evaporated  to  dryness 
in  iron  vessels,  and  the  solid  residue  was  used  for  the 
purposes  we  have  mentioned.  This  residue — impure 
potassium  carbonate — is  very  hygroscopic ;  it  rapidly 


COMPOUNDS   OF   POTASSIUM.  305 

attracts  moisture  from  the  air,  and  after  a  time  becomes 
a  liquid  ;  it  could  not,  therefore,  be  kept  in  bags  or 
casks,  but  it  was  necessary  to  pack  it  in  pots  which 
could  be  closed  securely.  Hence  the  name  pot-ashes. 

Besides  the  normal  potassium  carbonate,  K2CO3,  an 
acid  salt  exists,  produced  by  replacing  one  atom  of 
hydrogen  in  the  hypothetical  carbonic  acid  by  potassium. 
The  formula  of  this  salt  is  KHCO3 ;  it  is  called  potassium 
hydrogen  carbonate,  or  acid  carbonate  of  potassium,  or 
sometimes  bicarbonate  of  potash.  This  salt  is  produced 
by  passing  carbon  dioxide  into  a  concentrated  solution 
of  the  normal  carbonate  :  as  the  acid  salt  is  much  less 
soluble  than  the  normal  salt,  it  soon  begins  to  separate 
out  from  the  solution  into  which  carbon  dioxide  is 
passed. 

K2C03        f        C02        +      H20     =        2KHCO... 
Potassium  carbonic  acid  potassium 

carbonate  anhydride  carbonate. 

Potassium  carbonate  is  the  material  from  which 
caustic  potash  is  prepared.  Potash  lye,  as  a  solution 
in  water  of  caustic  potash  is  sometimes  called,  has 
been  used  for  centuries,  especially  in  the  preparation 
of  soap.  In  order  to  understand  the  preparation  of 
this  compound  from  potassium  carbonate,  we  must 
consider  for  a  moment  the  following  reactions.  We 
know  that  lime,  CaO,  is  produced  by  strongly  heating 
limestone  (p.  300).  When  water  is  added  to  lime,  the 
two  combine  to  form  slaked  lime  : — 

CaO     +     H20     =     Ca(OH)2. 

Lime      +     water     ==     slaked  lime. 

This  reaction,  like   many  other  chemical  reactions,  is 

20 


306      INTROPUCTION   TO   MODERN   CHEMISTRY. 

accompanied  by  the  production  of  heat.  In  this  case 
so  much  heat  is  produced  that  a  portion  of  the  water 
is  driven  off  as  steam.  When  one  who  is  unacquainted 
with  chemical  changes  sees  steam  produced  by  pouring 
cold  water  on  to  lime,  he  is  apt  to  think  there  is  some- 
thing mysterious  about  the  operation.  Slaked  lime  is 
a  white  powder  that  may  be  stirred  up  with  water ; 
when  it  is  treated  in  this  way  it  forms  the  lime-plaster 
used  by  masons.  But  slaked  lime  is  only  slightly 
soluble  in  water.  If  a  thin  milk  of  lime  (lime  stirred 
with  water)  is  filtered,  the  clear  filtrate  contains  one 
part  of  calcium  hydroxide  (or  slaked  lime)  dissolved  in 
760  parts  of  water.  Clear  lime-water  contains  but  a 
very  little  of  the  alkali  calcium  hydroxide,  but  thick 
milk  of  lime  is  very  rich  in  alkali. 

Milk  of  lime  is  used  for  making  caustic  potash  in 
the  following  way.  When  fairly  thin  milk  of  lime  is 
poured  into  a  solution  in  water  of  potassium  carbonate, 
the  calcium  hydroxide  seizes  the  carbon  dioxide  of  the 
carbonate,  and  calcium  carbonate  (which  is  insoluble  in 
water)  and  potassium  hydroxide  are  produced. 

K2CO3       +     Ca(OH)2     =       2KOH       +    CaCO3. 

Potassium  calcium  potassium  calcium 

carbonate  hydroxide  hydroxide  carbonate. 

If  a  concentrated  solution  of  potassium  carbonate  is 
decomposed  by  milk  of  lime  and  the  liquid  is  separated 
from  the  calcium  carbonate  which  precipitates,  a  con- 
centrated solution  of  potassium  hydroxide  (caustic 
potash)  is  obtained.  Solid  caustic  potash  is  produced 
by  evaporating  this  liquid  to  dryness,  and  heating  the 
residue  strongly  to  remove  the  whole  of  the  water. 


COMPOUNDS   OF   POTASSIUM.  307 

The  potash  is  melted  by  the  heat ;  on  cooling  it  remains 
as  a  hard,  -white  solid.  By  pouring  the  melted  potash 
into  moulds,  it  solidifies  in  the  form  of  sticks,  which 
are  convenient  for  use  in  the  laboratory.  Potassium 
hydroxide  is  a  strongly  caustic  substance  ;  hence  the 
name  caustic  potash. 

The  most  important  technical  application  of  solutions 
of  caustic  potash  in  water  is  in  making  soap.  All  soaps 
are  prepared  from  fats,  which  are  compounds  of  certain 
organic  acids  with  glycerin.  As  fats  are  compounds 
of  these  acids,  the  acids  are  grouped  together  under 
the  common  name  fatty  acids ;  they  all  contain  single 
chains  of  carbon  atoms.  The  acids  most  commonly 
found  in  soaps  are  stearic,  palmitic,  and  ole'ic  acid.  The 
fats  are  decomposed  by  boiling  with  potash  lye.  These 
glycerin  compounds  of  the  fatty  acids  react  with 
caustic  potash  to  form  potassium  compounds  of  the, 
fatty  acids  (which  are  soaps)  and  glycerin.  The  soaps 
produced  by  using  caustic  potash  are  soft  [hard  soaps 
are  obtained  by  using  caustic  soda]. 

Potassium  chloride  is  another  important  salt  of 
potassium ;  it  is  obtained  from  the  Stassfurt  deposits, 
and  finds  its  chief  application  as  an  artificial  manure. 

We  have  already  become  acquainted  with  potassium 
bromide  when  considering  bromine,  and  potassium 
iodide  when  considering  iodine. 

The  salt  potassium  chlorate,  which  we  used  when  we 
prepared  oxygen  gas,  is  formed  when  chlorine  gas  is 


308      INTRODUCTION   TO   MODERN    CHEMISTRY. 

passed  into  a  hot  solution  of  caustic  potash.  The 
chlorine  seizes  the  potassium  and  forms  potassium 
chloride.  The  hydroxyl  groups  (OH)  which  thus  be- 
come available  cannot  exist  alone  ;  they  must  enter 
into  some  reaction  whereby  stable  bodies  are  pro- 
duced. Six  of  these  groups  form  three  molecules  of 
water  and  three  atoms  of  oxygen  ;  and  the  three  atoms 
of  oxygen  react  with  a  molecule  of  potassium  chloride 
to  produce  potassium  chlorate,  which  separates  from 
the  liquid,  as  it  is  much  less  soluble  in  water  than 
potassium  chloride.  The  reaction  may  be  expressed 
in  the  following  equation  : — 

6KOH     +      6C1      =      KC1O3     +      $KC1      +  3H2O. 

Potassium  h  potassium         potassium 

hydroxide  chlorate  chloride 

We  already  know  something  about  potassium  nitrate 
and  potassium  nitrite  (p.  188). 

Potassium  silicate  is  obtained  by  the  reaction  between 
potassium  hydroxide  and  sand,  which  is  approximately 
pure  silica  (compare  p.  284).  This  salt  is  soluble  in 
water ;  it  is  sometimes  called  "  potash  soluble  glass." 
On  the  large  scale  the  salt  is  prepared  by  fusing  a 
mixture  of  potassium  carbonate  and  sand. 

Very  small  quantities  of  compounds  of  the  two 
metals  rubidium  and  ccesium  are  found  in  nature. 
These  metals  are  very  similar  chemically  to  potassium ; 
their  compounds  are  exceedingly  like  those  of  potassium 
both  in  composition  and  properties. 


PROPERTIES   OF   SODIUM.  309 


SODIUM. 

While  the  Stassfurt  mines  are  the  main  source  of 
potassium  compounds,  inexhaustible  supplies  of  com- 
pounds of  sodium  are  found  in  many  parts  of  the  earth. 
Vast  quantities  of  common  salt,  which,  as  we  know,  is 
sodium  chloride,  occur  in  salt  mines,  besides  what  is 
present  in  the  sea. 

Sodium  resembles  potassium  in  its  chemical  behaviour. 
Both  metals  can  be  prepared  by  similar  methods ; 
decomposition  of  sodium  hydroxide  by  the  electric 
current  produces  sodium,  just  as  potassium  is  obtained 
by  electrolysing  caustic  potash.  The  surface  of  freshly 
cut  sodium  is  silver-white.  Heated  in  dry  air,  sodium 
forms  Na2O2,  a  compound  which  is  called  sodium 
peroxide.  Hydrogen  peroxide  (p.  141)  is  obtained  by 
the  action  of  a  dilute  acid  on  this  compound  ;  for 
example : — 

Na202     +       2HC1        =       H202      +    2NaCl. 

Sodium          hydrochloric  hydrogen         common 

peroxide    *  acid  ~    peroxide  salt. 

Sodium  decomposes  water ;  hydrogen  is  set  free, 
and  the  sodium  combines  with  the  oxygen  of  the  water 
(compare  p.  30). 

In  the  course  of  time  methods  have  been  discovered 
for  transforming  sodium  chloride,  which  occurs  so 
plentifully  in  nature,  into  sodium  carbonate,  which  is 
scarcely  found  native.  Sodium  carbonate  has  long 
been  known  commercially  as  "  soda."  The  process 
whereby  "soda"  is  made  from  sodium  chloride  is 


3.1.0      INTRODUCTION    TO    MODERN    CHEMISTRY. 

exceedingly  ingenious ;  but  it  is  so  complicated,  both 
technically  and  chemically,  that  we  shall  not  consider 
it  in  this  INTRODUCTION  TO  CHEMISTRY.* 


Sodium  carbonate  (or  "  soda ")  is  very  similar  to 
potassium  carbonate  in  its  chemical  behaviour  :  but  the 
salts  differ  in  some  respects  ;  for  example,  the  potassium 
salt  is  very  hygroscopic,  and  it  is  so  soluble  in  water 
that  to  obtain  the  solid  salt  a  solution  of  it  must  be 
evaporated  to  dryness.  Sodium  carbonate  is  also  easily 
soluble  in  water,  but  it  is  much  more  soluble  in  hot 
water  than  in  cold.  If  a  solution  which  has  been  made 
by  saturating  hot  water  with  sodium  carbonate  is  allowed 
to  cool,  the  salt  separates  in  crystals  from  the  cold 
liquid.  The  "  soda  crystals "  prepared  in  this  way 
contain  ten  molecules  of  water  of  crystallisation ;  the 
formula  of  the  crystalline  compound  is  Na2CO3 .  ioH2O. 
Let  us  calculate  the  percentage  of  water  in  "  soda 
crystals."  The  atomic  weights  of  sodium,  carbon,  and 
oxygen  are  23,  12,  and  1 6  respectively — the  atomic 
weight  of  hydrogen  being  I  ;  hence  the  molecular 
weight  of  the  crystalline  salt  is  : — 

Na2  C  O3  10         H2O 

(2  x  23)  +  12  +  (sx  16)  -f  io([2  x  i]  +  16)  =  286. 

As  286  parts  by  weight  of  "  soda  crystals "  contain 
1 80  parts  of  water,  it  follows,  from  the  proportion 
286  :  1 80  =  100  :  x,  that  y  (that  is,  the  percentage  of 
water  in  the  crystals)  is  62-9.  Almost  two-thirds  of 

*  Details  of  this  process  are  given  in  Chemistry  in  Daily  Life, 
pp.  199-211  (2nd  Ed.). 


SODIUM   CARBONATE.  311 

the    weight    of  any    quantity    of    "soda    crystals"    is 
water. 

If  the  crystallised  sodium  carbonate  is  heated,  the 
water  is  driven  off,  and  the  anhydrous  "soda"  remains 
in  the  form  of  a  white  powder.  As  dry  "  soda " 
contains  much  more  sodium  carbonate  than  an  equal 
weight  of  "  soda  crystals/'  it  is  more  economical,  from 
considerations  of  the  cost  of  transport,  to  manufacture 
"  soda  "  than  "  soda  crystals."  When  a  compound  which 
contains  water  of  crystallisation  is  deprived  of  that 
water  by  heating,  the  process  is  called  calcination  ; 
hence  dry  "  soda  "  is  often  spoken  of  as  calcined  soda. 

The  water  cannot  be  seen  in  crystals  of  sodium 
carbonate.  If  the  crystals  are  exposed  to  the  air,  they 
gradually  change  to  a  white  powder.  The  water  of 
crystallisation,  which  is  present  in  such  large  quantity, 
slowly  evaporates,  as  any  other  water  would  do ;  and 
as  the  crystallised  form  is  dependent  on  the  presence  of 
the  ten  molecules  of  water,  the  crystals  crumble  to 
powder  when  the  water  is  removed.  The  loss  of  water 
of  crystallisation  is  called  the  efflorescence  of  crystals. 
We  must  not,  however,  suppose  that  all  compounds 
which  contain  water  of  crystallisation  effloresce ;  some 
of  them,  on  the  contrary,  retain  their  water  permanently 
at  ordinary  temperatures. 

Sodium  carbonate  may  be  used  in  place  of  potassium 
carbonate  for  many  purposes ;  is  is  often  employed 
nowadays  where  potassium  carbonate  was  once  used. 
The  latter  is  much  more  expensive  than  "  soda," 
because  "  soda "  can  be  manufactured  cheaply,  from 


312      INTRODUCTION    TO    MODERN    CHEMISTRY. 

common  salt.  If  we  wish  to  know  what  weight  of 
sodium  carbonate  is  required  to  replace  a  determinate 
weight  of  potassium  carbonate  in  performing  some 
chemical  reaction,  we  obtain  this  information  by  using 
the  following  proportion  :— 

K2C03  :  Na,CO3  =  i  :  x.     • 

The  molecular  weight  of  K2CO3  is  138  (K2  =  78  + 
C  =  12  -f  O3  =  48),  and  the  molecular  weight  of 
Na2CO3  is  1 06  (Na2  =  46  +  0=  12 +  0.3  =  48);  hence 
1 06  parts  of  sodium  carbonate  will  replace  138  parts  of 
potassium  carbonate.  Substituting  these  values  in  the 
proportion  already  given,  we  have  138  :  106  =  i  :  x ; 
hence  x  =  074.  That  is  to  say,  74  kilos,  of  sodium 
carbonate  will  replace  100  kilos,  of  potassium  carbonate. 

As  potassium  carbonate  combines  with  a  molecule  of 
carbonic  anhydride  and  forms  acid  potassium  carbonate 
when  the  gas  is  passed  into  a  solution  of  the  salt,  so  is 
acid  sodium  carbonate  produced  when  a  solution  of  the 
normal  salt  is  caused  to  react  with  carbonic  anhydride. 
Na2CO3  +  CO,  +  H20  =  2NaHCOs. 

Sodium  carbonate  +  carbonic  anhydride  +  water  —  aci  ,so  1"m 

The  reaction  between  calcium  hydroxide  (slaked 
lime)  and  a  solution  of  sodium  carbonate  is  similar 
to  that  between  calcium  hydroxide  and  a  solution  of 
potassium  carbonate  :  calcium  carbonate  is  formed,  and 
precipitates,  as  it  is  insoluble  in  water,  and  the  solution 
contains  sodium  hydroxide. 

Na2CO3   +  Ca(OH)2  =     CaCO3    +    2NaOH. 
Sodium  calcium  calcium  sodium 

.  carbonate        hydroxide  ~~  carbonate        hydroxide. 


CAUSTIC   SODA,  313 

By  removing  the  calcium  carbonate  by  filtration,  a 
solution  of  sodium  hydroxide,  or  caustic  soda,  is 
obtained.  Solid  caustic  soda  remains  as  a  white  mass 
when  the  solution  is  evaporated  to  dryness  and  the 
residue  is  heated.  Like  caustic  potash,  caustic  soda 
may  be  melted  and  cast  in  sticks. 

Much  use  is  made  of  caustic  soda  in  manufacturing 
processes;  thousands  of  hundredweights  of  it,  indeed, 
are  used  daily.  Fats  are  decomposed  by  caustic  soda  ; 
hence  it  is  employed  in  making  soaps.  The  process  is 
precisely  similar  to  that  which  occurs  when  caustic 
potash  is  used  : — 

Glycerin  compound    .    caustic  soda  _  sodium  salt  of         , 
of  fatty  acid  solution  fatty  acid 

(soap) 

Potash  soaps  are  soft ;  soda  soaps  are  hard. 

Caustic  soda  is  also  used  for  boiling  with  rags  in 
making  paper ;  also  for  decomposing  wood,  and  getting 
cellulose  therefrom,  in  the  same  manufacture.  Acid 
sulphite  of  calcium  is  a  keen  competitor  against  caustic 
soda  in  paper-making,  for  a  solution  of  this  salt  is  able 
to  break  up  wood  and  produce  cellulose.  The  formula 
of  this  salt  is  CaH2(SO3)2 :  sulphurous  acid  is  H2SO3  ; 
and  as  an  atom  of  calcium  is  divalent  and  takes  the 
place  of  two  atoms  of  hydrogen,  the  formula  of  normal 
calcium  sulphite  is  CaSO3.  This  salt  combines  with 
sulphurous  acid  to  form  the  acid  salt  CaSO3  .  H2SO3, 
or  CaH2(SO3)2. 

Caustic  soda  is  much  used  in  dyeing,  and  in  many 
other  chemical  industries. 


314      INTRODUCTION   TO   MODERN   CHEMISTRY. 

Of  the  other  salts  of  sodium,  mention  must  be  made 
of  the  sulphate.  The  formula  of  sodium  sulphate  is,  of 
course,  Na2SO4.  The  crystals  which  are  deposited  as 
a  hot  aqueous  solution  of  this  salt  cools  have  the 
composition  Na2SO4  .  ioH2O ;  the  crystalline  salt  is 
sometimes  called  Glauber's  salt,  after  the  name  of  its 
discoverer.  The  salt  is  still  prepared  by  the  method 
given  by  Glauber  more  than  two  hundred  years  ago ; 
sodium  chloride  is  decomposed  by  sulphuric  acid.  As 
a  fairly  high  temperature  is  needed  to  complete  the 
reaction,  the  two  substances  are  heated  together  in 
a  furnace. 

2NaCl      +       H2SO4       =        Na2SO4        +         2HC1. 
Common  salt  -t-  sulphuric  acid  =  sodium  sulphate  +  hydrochloric  acid. 

We  became  acquainted  with  this  reaction  when  we 
were  preparing  hydrochloric  acid ;  we  did  not  then 
pay  attention  to  the  sodium  sulphate  which  is  produced 
(see  p.  67). 

We  have  already  learnt  something  of  sodium  thio- 
sulphate  (p.  15!),  and  also  of  sodium  nitrate  and 
sodium  nitrite  (p.  188). 

Sodium  silicate  is  obtained  by  a  process  similar  to 
that  whereby  potassium  silicate  is  made.  Sodium 
hydroxide  is  caused  to  react  with  silica,  or  sodium 
carbonate  is  melted  with  sand.  The  salt  dissolves  in 
water;  it  is  commonly  called  "soda  soluble  glass." 

Borax  is  a  sodium  compound ;  it  is  a  sodium  salt 
of  boric  acid.  The  element  boron  (the  symbol  for 


BORAX.  3 1  5 

which  is  B)  is  generally  placed  among  the  non-metals, 
although  some  of  its  reactions  are  those  of  a  metal. 
It -is  a  trivalent  element;  it  is  able,  therefore,  to  hold 
to  itself  three  hydroxyl  groups,  and  to  form  B(OH)3,  a 
compound  which,  as  it  has  acidic  properties,  is  called 
boric  (or  boracic)  acid.  The  acidic  properties  of  this 
compound  are  very  feebly  marked  ;  it  does  not  form 
normal  salts  [the  normal  sodium  salt  would  be 
Na3BO3].  Borax  is  the  sodium  salt  of  an  acid  which 
has  the  composition  H2B4O7 ;  this  compound,  which 
is  called  pyroboric  acid,  may  be  regarded  as  formed 
by  removing  five  molecules  of  water  from  a  complex 
boric  acid,  thus  :— 

4B(OH)3  -  5H20  =  H2B407. 

Borax  is  used  in  soldering,  an  operation  which 
consists  in  causing  two  pieces  of  metal  to  adhere.  It 
is  necessary  that  the  surfaces  of  the  metals  to  be 
soldered  should  be  perfectly  clean  ;  the  slightest  film 
of  oxide  prevents  the  joining  of  the  metals.  When 
borax  is  strewed  on  the  metals  to  be  soldered  and 
they  are  heated  till  the  borax  melts,  that  salt  acts  like 
an  acid,  and,  dissolving  any  traces  of  oxides  that  may 
be  present,  insures  the  perfect  contact  of  the  metallic 
surfaces. 

The  metal  lithium,  and  also  the  group  ammonium 
(NH4),  are  chemically  closely  allied  to  sodium.  It  is 
not  necessary  for  us  to  consider  lithium.  We  have 
already  studied  ammonium  somewhat  fully  (p.  171). 

The    metals  potassium,    sodium,  rubidium,  caesium 


316      INTRODUCTION   TO    MODERN    CHEMISTRY. 

lithium  (and  ammonium)  are  commonly  classed  together 
under  the  name  alkali  metals.  The  metals  of  the  alkaline 
earths  are  allied  to  the  alkali  metals  ;  they  are  calcium, 
barium,  and  strontium.  It  is  only  the  first  of  these 
which  is  of  special  interest  to  us ;  the  two  others  are 
very  like  calcium  in  their  chemical  behaviours. 


CALCIUM. 

The  metal  calcium  is  obtained  by  the  electrolysis 
of  calcium  chloride,  CaCl2 ;  the  electric  current  decom- 
poses this  salt  into  calcium  and  chlorine  gas.  Calcium 
is  a  pale  yellow  metal,  somewhat  heavier  than  water; 
its  specific  gravity  is  i'6.  Calcium  oxidises  rapidly 
by  exposure  to  air ;  and  as  it  decomposes  cold  water, 
it  must  be  kept  under  a  liquid,  such  as  petroleum, 
•which  is  free  from  oxygen. 

Many  compounds  of  calcium  are  found  abundantly 
in  nature ;  we  know  that  limestone  and  marble  are 
calcium  carbonate  (p.  300) ;  alabaster  and  gypsum  are 
calcium  sulphate  ;  phosphorite  is  calcium  phosphate. 

Burnt  lime,  CaO,  is  obtained,  as  we  know,  by 
strongly  heating  limestone  (or  marble)  ;  lime  is  slaked 
by  adding  water  to  it,  whereby  calcium  hydroxide, 
Ca(OH)2,  is  produced  (p.  305).  Mortar  is  formed  by 
mixing  slaked  lime  and  sand  with  water  to  the  con- 
sistency of  a  paste.  Mortar,  laid  between  the  stones 
or  bricks  of  a  building,  hardens,  and  binds  the  stones 
together ;  during  the  hardening  process  carbon  dioxide 


MORTAR   AND   CEMENTS.  317 

is   absorbed   from    the    air,   and   calcium  carbonate  is 
formed. 

Ca(OH)2          +  CO2  CaCO3  +  H2O. 

Calcium  hydroxide   +    carbon  dioxide  =  calcium  carbonate  -f  water. 

We  notice  that  water  is  formed  in  this  reaction. 
Mortar  does  not  become  thoroughly  dry — does  not 
stop  exuding  water — until  the  whole  of  the  calcium 
hydroxide  in  it  has  been  changed  to  calcium  carbonate. 
Remembering  that  the  air  contains  only  '03  per  cent, 
of  carbon  dioxide  (by  volume),  we  can  understand  why 
the  walls  of  a  new  building  take  such  a  long  time  to 
become  dry.  The  drying  process  may  be  hastened 
by  burning  coke  in  the  rooms  of  a  newly  built  house 
while  the  doors  and  windows  are  closed.  The  air  of 
the  rooms  becomes  charged  with  carbon  dioxide,  and 
the  conversion  of  the  calcium  hydroxide  in  the  mortar 
into  calcium  carbonate  proceeds  more  rapidly. 

As  carbon  dioxide  is  necessary  for  the  hardening  of 
mortar,  ordinary  mortar  does  not  harden  under  water, 
because  there  is  very  little  carbon  dioxide  in  water. 
In  making  mortar  that  will  harden  under  water — such 
mortars  are  called  hydraulic  mortars,  or  cements — it  is 
necessary  to  burn  a  mixture  of  limestone  with  sand, 
and  clay,  which  is  a  silicate  of  aluminium  (see  under 
aluminium}.  If  such  a  mixture  is  burnt,  and  then 
ground  into  fine  powder,  and  mixed  with  water,  it 
hardens  under  water ;  calcium  silicate  and  calcium 
aluminate  (see  forward,  p.  327)  are  gradually  formed, 
and  the  formation  of  these  compounds  produces  a 
hard  mass.  Carbon  dioxide  is  not  required  for  the 
hardening  of  such  a  mixture. 


318      INTRODUCTION   TO   MODERN    CHEMISTRY. 

Calcium  chloride,  CaCl2,  is  obtained  by  dissolving 
calcium  hydroxide  (slaked  lime)  in  hydrochloric  acid, 
and  evaporating  the  solution  to  dryness.  The  salt  is 
thus  obtained  as  a  white  solid,  suitable  for  use  as  a 
drying  agent.  We  know  that  when  gases  are  passed 
over  this  salt  they  are  robbed  of  any  water  that  may 
be  mixed  with  them  (p.  138). 

Ca(OH)2         +  2HC1  CaCl2          +  2H2O. 

Calcium  hydroxide  +  hydrochloric  acid  =  calcium  chloride  +  water. 

Chloride  of  lime  is  quite  a  different  substance  from 
calcium  chloride.  It  is  prepared  by  leading  chlorine 
gas  over  calcium  hydroxide,  after  the  manner  described 
on  pp.  49  and  232,  where  we  became  acquainted  with 
the  composition  of  this  substance. 

An  aqueous  solution  of  chloride  of  lime  bleaches  slowly.  But 
the  full  bleaching  effect  of  such  a  solution  is  only  obtained  when 
acid  is  added  to  it.  The  acid  decomposes  the  substance,  and  sets 
free  the  chlorine  that  was  used  in  the  preparation  of  the  chloride 
of  lime,  and  this  chlorine  bleaches  (compare  p.  108).  If  hydro- 
chloric acid  is  used,  the  reaction  that  occurs  may  be  expressed 
thus  :— 

Ca(OCl)2    +       4HC1        =      4C1       +    CaCl2  +  2H2O. 

Calcium  hydrochloric         «t.i,«:«      ^  calcium    , 

hypochlorite   +          acid  +  chloride  +    water' 

Chloride  of  lime  contains  calcium  chloride,  CaCl2,  besides 
calcium  hypochlorite,  Ca(OCl)2.  The  hydrochloric  acid  reacts 
with  the  latter  compound  only. 

Calcium  sulphate,  CaSO4,  occurs  native.  When  this 
compound  is  found  without  water  of  crystallisation,  it  is 


MANUFACTURE  OF   GLASS.  319 

called  anhydrite ;  generally,  however,  it  occurs  with 
two  molecules  of  water  of  crystallisation,  and  is  called 
gypsum,  or  alabaster  when  it  has  a  peculiarly  fine  texture 
and  a  beautiful  surface.  When  gypsum  is  heated 
slightly  above  100°  C.  [212°  F.],  it  loses  its  water  of 
crystallisation.  If  this  burnt  gypsum  is  mixed  with 
water  to  the  consistency  of  a*paste,  it  re-combines  with 
two  molecules  of  that  compound.  The  paste  may  easily 
be  made  to  fill  a  mould  completely ;  it  hardens  rather 
rapidly,  re-forming  crystalline  gypsum.  This  process 
is  much  used  for  making  casts,  etc.  [calcium  sulphate 
used  for  such  purposes  is  generally  called  "  plaster  of 
Paris  "]. 

If  gypsum  is  too  strongly  heated,  it  will  not  re- 
combine  with  water ;  it  is  said  to  be  "  overburnt." 
Such  gypsum  is,  of  course,  useless  for  making  casts. 

We  must  now  glance  at  the  salt  calcium  silicate. 
This  compound  is  of  no  great  importance  in  itself; 
but  when  it  is  melted  with  certain  other  silicates, 
glass  is  produced,  which  is  an  exceedingly  unchange- 
able substance.  Chemically  considered,  glass  is  a 
double  silicate ;  ordinary  glass  generally  consists  of 
calcium  silicate  and  sodium  silicate,  because  these 
are  the  cheapest  materials  from  which  glass  can  be 
made. 

Glass  is  not  made  by  mixing  these  two  silicates  and 
melting  the  mixture ;  the  materials  used  are  sand 
(which  is  more  or  less  pure  silica,  SiO2),  calcium 
carbonate,  and  sodium  carbonate  ("soda").  These 
materials  are  mixed  and  heated  till  the  mixture  melts. 
The  silica,  which  is  non-volatile  at  high  temperatures, 


320      INTRODUCTION    TO   MODERN    CHEMISTRY. 

drives  out  the  carbonic  anhydride  of  the  two  carbonates 
when  the  mixture  is  strongly  heated  in  a  furnace,  and, 
combining  with  the  calcium  and  the  sodium,  forms 
calcium  silicate  and  sodium  silicate.  The  substance 
we  call  glass  is  thus  formed  in  the  furnace  directly  from 
the  materials  placed  therein. 


Sodium  sulphate,  which  is  cheaper  than  "soda,"  is  much  used 
in  glass-making,  instead  of  sodium  carbonate.  If  this  salt  is 
employed,  it  is  necessary  to  add  carbon  to  the  mixture  in  the 
furnace.  The  reactions  proceed  as  follows.  The  carbon  reduces 
the  sodium  sulphate  to  sodium  sulphite,  and  is  itself  oxidised  to 
carbon  monoxide  gas,  which  passes  off. 

Na,S04        +       C     r-        Na2SO3        +  CO. 

Sodium  sulphate  +  carbon  =  sodium  sulphite  +  carbon  monoxide  gas. 

The  silica  then  reacts  with  the  sodium  sulphite ;  sodium 
silicate  is  produced,  while  sulphur  dioxide  (which  is  a  gas,  see 
p.  149)  escapes  from  the  furnace.  Although  a  sulphur  compound 
(sodium  sulphate)  was  put  into  the  furnace,  the  glass  contains 
no  sulphur,  nor  does  it  contain  any  of  the  carbon  that  was  added 
to  reduce  the  sodium  sulphate  to  sulphite. 


If  potassium  carbonate  is  used  in  place  of  sodium 
carbonate  in  glass-making,  a  double  silicate  of  calcium 
and  potassium  is  produced.  Such  potash  glass  is  much 
less  easily  melted  than  soda  glass  ;  it  is  used  for  special 
purposes  by  chemists  and  glass-cutters. 

Lead  glass  is  obtained  by  melting  mixtures  of  lead 
oxide  and  sodium  carbonate  with  silica.  This  glass  is  a 
double  silicate  of  lead  and  calcium ;  it  is  very  lustrous 
when  cut,  and  is  sometimes  used  for  making  artificial 
precious  stones. 


PROPERTIES  OF   MAGNESIUM.  321 


MAGNESIUM. 

Magnesium  is  a  member  of  the  group  of  metals  beryllium, 
magnesium,  zinc,  cadmium.  Beryllium  is  one  of  the  rare  ele- 
ments ;  its  most  commonly  occurring  compound  is  beryl,  which 
is  a  double  silicate  of  beryllium  and  aluminium.  If  this  com- 
pound is  coloured  green  by  the  presence  of  a  very  little  oxide  of 
chromium,  it  forms  the  precious  stone  called  emerald.  Zinc  and 
cadmium,  which  belong  to  the  same  group  as  magnesium,  are 
heavy  metals,  so  that  magnesium  forms  a  kind  of  stepping  stone 
from  the  heavy  to  the  light  metals. 


Magnesium  compounds  are  found  very  abundantly 
in  nature  ;  they  are  generally  associated  with  com- 
pounds of  calcium.  Magnesium  carbonate  is  called 
tnagnesite  by  mineralogists.  Various  ranges  of  moun- 
tains are  formed  of  dolomite,  which  is  a  double 
carbonate  of  magnesium  and  calcium.  Almost  all 
naturally  occurring  waters  contain  compounds  of  mag- 
nesium, besides  compounds  of  calcium.  Magnesium 
phosphate  is  found,  along  with  calcium  phosphate,  in 
the  bones  of  all  animals. 

Magnesium  is  obtained,  like  the  other  light  metals, 
by  the  electrolysis  of  its  chloride  ;  the  electric  current 
decomposes  that  compound  into  magnesium  and  chlorine. 
Magnesium  is  a  silver-white,  lustrous  metal;  its  specific 
gravity  is  only  I "].  The  metal  is  easily  set  on  fire : 
in  burning  it  emits  extremely  white  light;  for  this 
reason  magnesium  powder  is  used  for  taking  instan- 
taneous photograph's  in  darkened  rooms.  As  mag- 
nesium does  not  react  with  cold  water,  it  may  be 
kept  under  water,  unlike  sodium  and  potassium.  [Hot 

21 


322      INTRODUCTION   TO   MODERN    CHEMISTRY. 

magnesium,  however,  decomposes  steam,  forming  mag- 
nesium oxide  and  hydrogen.] 

Magnesium  oxide >  MgO,  may  be  obtained  by  strongly 
heating  magnesium  carbonate,  as  calcium  oxide  is 
obtained  by  strongly  heating  calcium  carbonate. 

MgCO3  MgO          +          CO2. 

Magnesium  carbonate  =  magnesium  oxide  +  carbon  dioxide. 

CaCO3  CaO  +          CO., 

Calcium  carbonate      =      calcium  oxide      +  carbon  dioxide. 

Magnesium  oxide  is  generally  called  magnesia,  a 
name  which  has  come  down  from  former  times ;  in 
pharmacy  it  is  known  as  magnesia  usta  (or  burnt 
magnesia,  from  the  method  of  its  preparation).  If 
magnesium  carbonate  is  not  "  overburnt,"  the  oxide 
that  is  formed  combines  with  water,  just  as  calcium 
oxide  does,  and  forms  magnesium  hydroxide. 

MgO  +  H,O   =         Mg(OH)2. 

Magnesium  oxide  +  water  —  magnesium  hydroxide. 

Large  quantities  of  magnesium  chloride,  MgCl2,  are 
found  in  the  Stassfurt  deposits,  along  with  calcium 
chloride  (compare  p.  24).  Magnesium  sulphate  has 
long  been  known  under  the  name  of  bitter  salt,  because 
of  its  bitter  taste.  [In  this  country  it  is  often  called 
Epsom  salts, ,]  It  is  found  in  many  waters ;  at  Stassfurt 
it  occurs  in  large  quantities.  A  great  number  of  mag- 
nesium silicates  is  known.  Talc  and  meerschaum  are 
silicates  of  magnesium  ;  asbestos,  which  is  used  for  its 
fire-resisting  property,  is  a  double  silicate  of  magnesium 
and  calcium. 


PREPARATION  OF  ALUMINIUM.  323 


ALUMINIUM. 

The  last  metal  we  shall  consider  is  aluminium. 
Silicates  of  aluminium  are  found  in  the  earth  in  vast 
quantities,  forming  the  various  kinds  of  clay.  The 
im  purer  clays  —  those  which  contain  iron  and  sand  —  are 
used  for  making  bricks  and  pottery  ;  better  varieties  are 
employed  in  the,  manufacture  of  stoneware  and  majolica- 
ware  ;  and  the  purest  clays  are  used  in  making 
porcelain.* 

Compounds  of  aluminium  are  evidently  very  widely 
distributed  ;  but  it  is  only  in  comparatively  recent 
times  that  the  metal  has  been  prepared.  Of  course,  it 
has  long  been  possible  to  prepare  silica  and  aluminium 
oxide  from  clay  —  that  is,  from  aluminium  silicate.  But 
no  method  was  found  for  decomposing  aluminium  oxide 
(compare  what  was  said  regarding  calcium  oxide  on 
p.  300). 

Wohler  isolated  aluminium  in  1827,  by  heating  the 
chloride  of  the  metal  with  sodium. 


A1C1S  +    sNa    =        Al        + 

Aluminium  chloride  +  sodium  =  aluminium   -t-  sodium  chloride. 

This  method  is  not  suited  for  the  cheap  production 
of  aluminium,  because  the  preparation  of  anhydrous 
aluminium  chloride  is  a  troublesome  affair.  There 
would  be  no  difficulty  in  preparing  this  chloride  could 
it  be  made  by  a  method  similar  to  those  whereby 

*  More  details  concerning  these  various  wares  will  be  found 
in  Chemistry  in  Daily  Life,  pp.  235-243  (2nd  Ed.). 


324    INTRODUCTION  TO  MODERN  CHEMISTRY. 

we  prepared  sodium  chloride  and  calcium  chloride— 
that  is  to  say,  by  dissolving  the  oxide  or  hydroxide  of 
the  metal  in  hydrochloric  acid,  evaporating,  and  crystal- 
lising. It  is  easy  to  obtain  aluminium  oxide ;  the 
mineral  bauxite  contains  this  oxide  [and  ferric  oxide]. 
If  aluminium  oxide  is  dissolved  in  hydrochloric  acid, 
aluminium  chloride  is  formed,  as  one  would  expect. 

A1203  +  6HC1  2A1C13  +  3H,O. 

Aluminium  oxide  +  hydrochloric  acid  =  aluminium  chloride  +  water. 

But  when  the  solution  is  evaporated,  the  aluminium 
chloride  and  the  water  react  to  re-form  aluminium 
oxide  and  hydrochloric  acid,  so  that  anhydrous  chloride 
of  aluminium  cannot  be  obtained  by  this  method. 

2A1C13  +  3H2O=          A12O3          +          6HC1. 

Aluminium  chloride  +  water  =  aluminium  oxide  +  hydrochloric  acid. 

In  order  to  obtain  anhydrous  aluminium  chloride, 
chlorine  gas  is  passed  over  a  mixture  of  aluminium 
oxide  and  carbon  (charcoal)  while  the  mixture  is  kept 
red  hot. 

A12O3       +      30      +      6C1       *      2A1C13      +        sCO. 

Aluminium         carbon         chlorine    _    aluminium  carbon 

oxide  chloride  monoxide  gas. 

Aluminium  oxide  is  not  reduced  by  carbon,  even  at 
the  highest  attainable  temperature ;  if  it  were  reduced, 
this  would  be  the  most  convenient  method  of  preparing 
aluminium  (as  lead  is  obtained  by  heating  its  oxide 
with  carbon).  But  when  chlorine  is  passed  over  a 
strongly  heated  mixture  of  aluminium  oxide  and  carbon, 
the  tendency  of  aluminium  to  combine  with  chlorine 


PREPARATION   OF   ALUMINIUM.  325 

is  added  to  the  striving  of  the  carbon  to  remove  oxy- 
gen from  the  oxide :  the  result  of  this  double  attraction 
is  the  combination  of  the  carbon  with  the  oxygen  of 
the  aluminium  oxide  to  form  carbon  monoxide,  and 
the  combination  of  the  chlorine  with  the  aluminium  to 
form  aluminium  chloride.  As  all  the  materials  are 
dry  and  are  raised  to  a  high  temperature,  the  chloride 
of  aluminium  that  is  produced  is  anhydrous.  When 
this  salt  is  heated  with  sodium,  in  the  manner  practised 
seventy  years  ago,  metallic  aluminium  is  produced. 

We  know  that  sodium  can  be  obtained  electro- 
lytically  (p.  301.)  Instead  of  causing  sodium  to  react 
with  anhydrous  aluminium  chloride,  which  is  costly 
to  prepare,  aluminium  is  prepared  to-day  by  a  direct 
electrolytic  method,  similar  to  that  whereby  sodium 
can  be  obtained. 

If  aluminium  oxide  could  be  melted,  it  would  only 
be  necessary  to  pass  an  electric  current  through  the 
melted  substance  in  order  to  obtain  aluminium ; 
oxygen  would  also  be  formed,  but  that,  being  a  gas, 
would  pass  off.  This  process  cannot  be  applied 
directly,  because  aluminium  oxide  does  not  melt  at  the 
highest  attainable  temperature.  But  it  is  possible  to 
produce  a  fusible  substance  by  mixing  aluminium  oxide 
with  fluorspar  (calcium  fluoride,  see  p.  76)  and  similar 
materials.  This  mixture,  if  properly  made,  melts  at 
the  temperature  of  the  electric  furnace ;  and  at  the 
same  time  the  aluminium  oxide  is  separated,  by  the 
electric  current  which  flows  through  the  furnace,  into 
aluminium  and  oxygen.  (For  a  description  and  repre- 
sentation of  the  electric  furnace,  see  p.  275). 


326      INTRODUCTION    TO   MODERN    CHEMISTRY. 

It  is  by  the  application  of  this  method  that  aluminium 
has  been  produced  at  a  moderate  cost  in  recent  years. 
The  hopes  that  aluminium  would  take  the  place  of 
certain  other  metals,  because  of  its  low  specific  gravity, 
have  not  been  realised  so  fully  as  was  anticipated. 
The  want  of  hardness  and  rigidity  of  the  metal  neces- 
sitates the  use  of  comparatively  large  quantities  of  it, 
so  that  what  might  be  gained  in  lightness  is  more  than 
counterbalanced  in  other  ways. 

We  learnt  something  of  the  alums,  which  are 
compounds  of  aluminium  (the  name  of  the  metal  being 
derived  from  that  of  the  compound),  when  we  were 
considering  the  double  salts.  The  alum  which  has 
been  known  for  the  longest  time  is  potash  alum — 
potassium  aluminium  sulphate,  K2SO4 .  A12(SO4)3 .  24H2O 
(see  p.  1 60). 

Alumstone  is  found  in  certain  parts  of  the  earth — 
near  Rome,  for  example;  if  this  mineral  is  heated, 
and  then  lixiviated  with  hot  water,  alum  crystallises 
from  the  solution  as  it  cools.  This  is  the  oldest 
method  for  making  alum ;  the  process  is  easily  con- 
ducted, and,  as  alum  crystallises  very  readily,  there 
is  no  difficulty  in  purifying  the  product.  Alum  has 
been  used  in  dyeing  for  a  very  long  time.  The  goods 
are  soaked  in  an  alum  bath,  and  the  alumina  in 
the  alum  causes  the  colour  to  be  held  fast  in  the 
fibres  of  the  fabric.  Most  colours  show  no  tendency 
to  adhere  to  the  fibres  of  textile  fabrics ;  the  colours 
can  be  washed  out  by  water.  But  if  the  fabric  is 
dipped  in  a  solution  containing  alumina,  that  compound 
js  deposited  in  the  fibres ;  and  when  the  fabric  is  then 


PREPARATION    OF   ALUM.  327 

immersed  in  a  solution  of  a  colouring  substance,  the 
colour  enters  into  union  with  the  alumina,  and  is  so 
firmly  held  that  it  cannot  be  removed  by  washing  with 
water. 

It  should  be  remarked  that,  although  alumina  is  the  most 
important  mordant,  it  is  by  no  means  the  only  one  in  use.  The 
action  of  all  mordants  is  similar  to  that  of  alumina. 

Alum  is  manufactured  to-day,  on  the  large  scale,  by 
other  methods.  As  alum  is  valuable  only  because  of 
the  aluminium  compound  it  contains,  and  the  potas- 
sium sulphate  in  it  is  unnecessary  and  increases  its 
cost,  pure  aluminium  sulphate  is  prepared  (and  the 
price  is  not  great,  as  sulphuric  acid  is  so  cheap)  and 
used  as  a  substitute  for  alum.  To  prepare  this  salt, 
a  silicate  of  aluminium  (see  p.  323),  which  is  found 
native  in  a  state  of  purity,  is  heated  with  sulphuric 
acid ;  sulphate  of  aluminium  is  formed,  and  silica 
remains  undissolved.  The  mixture  is  then  treated 
with  hot  water :  aluminium  sulphate  dissolves,  and 
silica  remains ;  the  liquid  is  filtered,  and  evaporated 
to  dryness. 

There  are  many  other  salts  of  aluminium,  but  most 
of  them  are  of  no  special  importance  for  our  purposes. 
We  must,  however,  say  something  regarding  calcium 
aluminate,  which  we  mentioned  when  considering 
cement  (see  p.  317).  We  know  that  alumina  is  a  very 
weak  base ;  for  instance,  when  a  solution  of  the  oxide 
in  hydrochloric  acid  is  evaporated,  the  aluminium 
chloride  in  the  solution  is  decomposed.  If  all  bases 
are  to  be  looked  on  as  hydroxyl  compounds  (compare 


328      INTRODUCTION   TO   MODERN   CHEMISTRY. 

p.  203),  we  must  think  of  aluminium  oxide  as  combining 
with  water  and  forming  the  base  A12O3  .  H2O,  or 
A12O2(OH)2.  This  hydroxide  reacts  .with  strong  bases 
as  a  weak  acid.  (Tin  dioxide  behaves  similarly  ;  see 
p.  300.)  If  we  suppose  that  the  two  hydrogen  atoms 
in  this  hydroxide  are  replaced  by  an  atom  of  calcium 
(which  is  a  divalent  element),  we  obtain  the  compound 
Al2O4Ca.  Such  compounds  are  called  aluminates.  This 
calcium  aluminate  is  found  in  cement,  after  that  is 
treated  with  water,  and,  along  with  calcium  silicate, 
causes  the  hardening  of  the  cement. 

The  precious  stone  chrysoberyl  is  composed  of 
beryllium  aluminate,  Al2O4Be.  Sodium  aluminate, 
Al2O4Na2,  and  potassium  aluminate,  A12O4K2,  are  also 
known ;  the  formulae  of  these  salts  are  generally 
halved,  and  written  AlO2Na  and  A1O2K  respectively. 


THE   SYSTEMATIC   ARRANGEMENT   OF   THE 
ELEMENTS. 

WE  have  now  become  acquainted  with  the  principal 
elements,  and  their  chemical  behaviour  in  broad  outline. 
The  general  conclusions  and  the  theories  we  deduced 
from  that  survey  led  to  a  system,  an  arrangement,  of 
all  the  material  things  around  us  considered  from  the 
chemical  point  of  view.  What  a  wonderful  order  was 
introduced  when  the  facts  were  regarded  in  the  light  of 
the  theory  of  atoms  and  molecules  ! 

We  must  now  inquire,  in  conclusion,  whether  it  is 
necessary  to  suppose  that  the  elements  are  particular 
kinds  of  matter,  each  quite  distinct  from  the  others. 
We  have  found  that  there  are  resemblances  between 
some  of  the  elements ;  we  noticed  certain  groups,  each 
of  four  similar  elements :  we  recall  chlorine,  bromine, 
iodine,  and  fluorine;  and  oxygen,  sulphur,  selenion, 
and  tellurium. 

The  question  we  have  suggested  is  so  natural  that 
it  must  have  engaged  the  attention  of  chemists  long 
ago.  In  the  year  1826  Prout  suggested  that  hydrogen 
is  the  foundation  of  all  the  elements;  that  the  others 
are  only  condensed  hydrogen,  and  have  been  formed 

3*9 


330      INTRODUCTION   TO   MODERN    CHEMISTRY. 

by  the  coalescence  of  atoms  of  hydrogen  to  produce 
larger  groups  of  atoms.  But  it  was  made  evident  in 
the  sixties  of  the  nineteenth  century  that  the  hypothesis 
of  Prout  was  not  in  keeping  with  fact?,  however  simple 
it  might  seem  when  it  was  not  inquired  into  too 
narrowly.  If  that  hypothesis  was  correct,  the  atomic 
weights  of  all  the  elements  must  be  whole  numbers, 
multiples  of  the  atomic  weight  of  hydrogen,  because 
we  are  bound  to  regard  the  atoms  of  hydrogen  as 
indivisible.  But,  by  using  extremely  fine  methods  and 
delicate  balances,  it  was  shown  to  be  impossible  to 
express  the  atomic  weights  of  all  elements  by  whole 
multiples  of  the  atomic  weight  of  hydrogen  taken  as 
unity.  The  atomic  weight  of  chlorine,  for  instance,  is 
35-5,  that  of  silver  is  107-9,  and  so  on. 

After  the  abandonment  of  Prout's  hypothesis,  the 
question  of  the  interdependence  of  the  elements  re- 
mained at  rest  for  some  time ;  it  was,  however,  again 
approached  from  different  sides  almost  simultaneously, 
and  it  has  at  last  found  its  final  [?]  expression  in 
the  table  of  Mendelejeff.  That  table  appears  on 
pp.  338  and  339.  Both  the  full  names  and  the  symbols 
of  the  elements  are  given,  as  the  latter  render  easier 
a  general  survey  of  the  arrangement,  and  because  the 
symbols  and  the  names  of  many  elements  have  not 
been  given  in  this  book,  except  in  the  tables  on  pp.  22 
and  88,  and  there  are  many  who  are  not  familiar  with 
all  the  elementary  symbols. 

This  table  is  drawn  up  on  what  seems  the  simplest 
principle.  As  we  see,  the  elements  are  arranged  one 
after  the  other  in  the  order  of  the  increasing  values  of 


CLASSIFICATION    OF   THE   ELEMENTS.          331 

their  atomic  weights.  It  must  not  be  forgotten  that 
we  do  not  yet  know  all  the  elements.  Hence  it  was 
necessary  to  leave  spaces  for  elements  yet  to  be 
discovered,  and  to  know  exactly  where  such  spaces 
should  be  left  if  the  continuity  of  the  system  was  to 
be  brought  into  proper  distinctness. 

Considering  this  arrangement  of  the  elements  in  the 
order  of  their  atomic  weights  as  a  whole,  we  notice, 
following  the  seventh  member  of  a  series,  an  eighth 
element,  which  therefore  begins  a  new  series,  and  has 
a  marked  similarity  with  the  first  member  of  the  series 
which  precedes  that  whereof  the  eighth  element  is  the 
first  member.  Newlands  had  remarked  this  before 
Mendelejeff;  and  this  circumstance  led  him  to  speak 
of  the  law  of  octaves  of  the  elements,  an  elegant  but  not 
particularly  helpful  expression. 

Hydrogen  does  not  fit  well  into  the  system ;  we 
must  assign  an  exceptional  position  to  this  element. 
The  same  thing  must  be  said  of  helium,  an  element 
recently  discovered  and  not  yet  sufficiently  examined. 
(We  have  already  [p.  136]  laid  stress  on  the  peculiar 
position  of  this  element).  Taking  series  2  and  3, 
it  is  certain  that  the  character  of  the  elements 
therein  changes  regularly  and  gradually  as  their  atomic 
weights  increase,  and  also  that  the  character  of  these 
elements  changes  periodically — that  is  to  say,  it  rises 
and  falls  in  much  the  same  way  in  the  two  series, 
so  that  corresponding  members  of  these  series  are 
analogous,  and  their  chemical  behaviours  are  similar. 
For  instance,  lithium  is  very  like  sodium  ;  both  are 
light  metals,  both  are  monovajent,  and  so  on.  We 


332      INTRODUCTION    TO   MODERN   CHEMISTRY. 

have  already  considered  in  detail  the  marked  similarity 
between  carbon  and  silicon  (p.  284)  ;  both  are  tetra- 
valent.  Not  less  marked  is  the  likeness  between 
oxygen  and  sulphur ;  both  are  divalent,  etc.  Because 
of  their  equal  valencies,  the  corresponding  members  of 
these  two  series  produce  compounds  of  the  same  form, 
as  is  indicated  by  the  formulae  of  oxides  placed  at  the 
heads  of  the  several  groups.  This  similarity  of  pro- 
perties of  the  elements  of  the  several  groups  is  certainly 
remarkable,  for  the  table  is  arranged  solely  on  the 
basis  of  the  atomic  weights  of  the  elements,  and  not, 
for  instance,  in  accordance  with  their  valencies.  Never- 
theless, the  elements  fall  into  their  proper  positions  as 
regards  their  valencies.  One  would  not  suppose  that 
atomic  weight  is  connected  with  valency ;  but  the 
table  shows  that  such  a  connection  exists. 

One  circumstance  is  of  special  importance  :  when  the 
oxides  and  hydrides  are  considered  (the  formulae  of 
these  compounds  are  placed  at  the  heads  of  the  several 
groups  in  the  table),  such  regularities  are  noticed  in 
the  forms  of  the  compounds,  in  many  series,  in  passing 
from  one  element  to  the  next  in  the  series,  that  it  is 
necessary  to  conclude  that  such  regular  series  are 
complete — that  all  the  elements  belonging  to  them  are 
already  known.  The  seven  elements  of  the  third 
series,  for  instance,  form  the  following  oxides : — 

Na20   Mg202(MgO)     A12O3    Si2O4(SiO2)     P2O5      S2O6(SO3)  (?C12O7) 

Sodium    Magnesium  Aluminium    Silicon       Phosphorus  Sulphur    (?Chlorine 

oxide.          oxidec          '  oxide.         oxide.  oxide.          oxide.         oxide.) 

We  see  that  the  quantity  of  oxygen  in  these  oxides 
in  creases  regularly  in  passing  from  the  first  to  the 


CLASSIFICATION   OF  THE   ELEMENTS.          333 

seventh  member  of  the  series.*    The  four  last  members 
of  this  series  form  the  following  hydrogen  compounds : — 

SiH4  PH3  SH,  C1H 

Silicon  hydride.      Phosphorus  hydride.    Sulphur  hydride.     Chlorine  hydride. 

If  we  inquire  into  the  chemical  properties  of  these 
four  hydrides,  we  find  that  chlorine  hydride  is  a  strong 
acid,  and  is  not  readily  decomposed  by  heat;  that 
sulphur  hydride  is  a  very  weak  acid,  which  is  decom- 
posed at  a  red  heat ;  that  phosphorus  hydride  has  no 
acidic  characters,  but  is  decomposed  at  a  lower  tem- 
perature than  sulphur  hydride ;  and  that  silicon  hydride 
is  not  acidic,  and  is  easily  separated  by  heat  into 
silicon  and  hydrogen. 

The  elements  in  the  several  series  exhibit  a  regular 
continuity,  not  only  as  regards  the  forms  of  certain 
compounds,  but  in  all  their  chemical  and  physical 
properties.  The  metals  come  towards  the  beginnings 
of  the  series,  and  the  non-metals  towards  the  ends ; 
for  instance,  the  third  series  begins  with  sodium  and 
ends  with  chlorine,  and  copper  and  bromine  are  at  the 
beginning  and  the  end  respectively  of  the  fifth  series. 
As  an  example  of  regularity  in  physical  properties,  we 
may  take  the  specific  gravities  of  the  elements  in  the 
seventh  series  : — 

Specific  gravity    .     10*5  8'6  7*4        7*2          6'j  6'2  4*9 

Name  of  Element.  Silver.  Cadmium.  Indium.  Tin.  Antimony.  Tellurium.  Iodine. 

*  I  have  bracketed  chlorine  oxide,  C12O7,  and  put  a  query 
before  it,  because  this  oxide  has  not  yet  been  isolated  with 
certainty,  although  the  corresponding  acid  (H.,C1.,O8)  is  known. 

[TR.] 


334      INTRODUCTION   TO   MODERN    CHEMISTRY. 

A  careful  consideration  of  the  table  shows  that,  when 
the  elements  are  arranged  in  periods  of  seven,  there  is 
always  a  marked  difference  between  the  last  member  of 
the  series  with  even  numbers  and  the  first  member 
of  the  odd-numbered  series — between  manganese  and 
copper,  for  instance.  Moreover,  all  those  elements 
whose  atomic  weights  and  properties  prevent  their 
being  placed  in  the  periods  of  seven  elements — these 
are  called  short  periods — are  found  between  the  last 
members  of  the  series  with  even  numbers  and  the  first 
members  of  the  series  with  odd  numbers.  For  instance, 
the  three  elements  iron,  nickel,  and  cobalt  are  found 
at  the  end  of  the  fourth  series — that  is,  between 
manganese  and  copper — and  ruthenium,  rhodium,  and 
palladium  come  at  the  end  of  the  sixth  series.  When 
we  take  these  elements  into  account,  we  obtain  the 
long  periods  of  seventeen  elements  each  [for  instance, 
the  long  period  beginning  with  potassium  and  ending 
with  bromine,  and  the  other  long  period  beginning  with 
rubidium  and  ending  with  iodine].  The  similarities 
between  the  corresponding  members  of  these  long 
periods  are  greater  than  those  between  the  corre- 
sponding members  of  the  short  periods.*  Potassium, 
rubidium,  and  caesium  ought  to  be  very  much  alike  ; 
on  p.  315  we  learnt  that  these  three  elements  are  very 
similar.  The  table  indicates  that  close  resemblances 
should  exist  between  sulphur,  selenion,  and  tellurium, 
and  between  chlorine,  bromine,  and  iodine;  we  know 
that  the  elements  in  each  of  these  triplets  are  very 
closely  allied. 

*  It  is  easy  to  overlook  the  arrangements  of  the  long  periods 
when  the  table  is  inspected. 


CLASSIFICATION   OF  THE   ELEMENTS.          335 

The  elements  in  the  eighth  group,  which  seems  to 
stand  somewhat  apart,  show  strongly  marked  similari- 
ties ;  they  are  all  exceedingly  infusible,  and  the  only 
elements  which  form  oxides  of  the  form  RO4  belong 
to  this  group. 

We  have  seen  that  conclusions  can  be  drawn 
regarding  certain  properties  of  elements  from  the 
positions  occupied  by  these  elements  in  the  table. 
Hence  it  is  possible  in  this  way  to  declare  the 
properties  of  an  element  which  has  not  been  isolated  ; 
for  these  properties  must  correspond,  on  the  whole, 
with  those  of  the  element  which  is  separated  from  it 
by  a  long  period  of  'seventeen  members.  MendelejefT 
established  the  possibility  of  doing  this.  The  names 
of  three  elements  are  printed  in  the  table  in  heavy 
type  ;  these  three  elements  have  been  isolated  since 
the  publication  of  Mendelejeff 's  memoir  in  1869. 
Mendelejeff  applied  his  table  in  this  way,  and  in  his 
first  memoir  gave  a  detailed  account  of  the  properties 
of  elements  which  were  then  unknown.  As  it  would 
have  been  somewhat  hazardous  to  have  given  names 
to  unknown  elements,  which  might  perhaps  never  be 
discovered,  as  they  might  not  be  found  anywhere  on 
the  earth's  surface,  Mendelejeff  designated  his  unknown 
elements  by  the  names  of  those  to  which  they  ought  to 
show  the  closest  analogies,  placing  the  Sanscrit  prefixes 
eka  and  dui  before  the  names  of  the  analogues  of  the 
undiscovered  elements.  The  element  whose  properties 
he  announced  in  the  greatest  detail  was  that  which  is 
placed  in  the  fifth  series  and  the  third  group,  and  was 
called  by  him  eka-aluminium.  In  the  year  1875  Lecoq 


336     INTRODUCTION   TO   MODERN   CHEMISTRY. 

de  Boisbaudran  discovered  a  new  element,  which  he 
named  gallium.  This  element  was  obtained  from  a 
specimen  of  zinc  blende  ;  and  as  the  mineral  contained 
only  very  small  traces  of  the  new  element,  the  first 
accounts  of  it  given  by  its  discoverer  were  not  very 
exact.  Mendelejeff,  however,  at  once  recognised  that 
the  new  element  was  his  eka-aluminium.  Thereupon 
he,  living  in  St.  Petersburg,  described  very  accurately 
the  properties  of  this  element,  which  had  been  in  the 
hands  of  only  one  chemist,  in  Paris.  As  the  element 
followed  zinc  in  the  table,  Mendelejeff  assigned  to 
its  atomic  weight  the  approximate  value  of  68.  As 
aluminium  oxide  has  the  formula  A12O3  (p.  324),  gallium 
oxide  must  be  Ga2O3 ;  and  as  the  atom  of  gallium  must 
be  trivalent,  since  the  element  belongs  to  Group  III., 
the  formula  of  its  chloride  must  be  Ga2Cl6  (or  GaG3). 
As  this  chloride  must  be  composed  of  68  parts  by 
weight  of  gallium  (the  atomic  weight  of  the  element 
being  68),  and  3x35*5  =  106*5  parts  of  chlorine,  the 
chloride  must  contain  39  per  cent,  of  gallium  and  61  per 
cent,  of  chlorine.  In  this  way  Mendelejeff  was  able  to 
announce  the  quantitative  composition  of  a  compound 
of  an  element  before  that  compound  had  been  seen  by 
anyone. 

From  the  position  of  the  new  element  in  the  table, 
it  follows  that  sulphuretted  hydrogen  must  precipi- 
tate sulphide  of  gallium  from  solutions  of  salts  of  the 
metal,  and  also  that  gallium  will  not  be  oxidised  by 
exposure  to  the  air.  Mendelejeff  also  predicted  the 
specific  gravity  of  this  element;  he  gave  the  value  5-9, 
which  is  almost  exactly  the  true  value.  The  fuller 
examination  of  gallium,  after  it  had  been  obtained  in 


CLASSIFICATION    OF   THE   ELEMENTS.          337 

larger  quantities,  confirmed  the  accuracy  of  everything 
that  had  been  predicted  by  Mendelejeff  with  regard  to 
its  physical  and  chemical  properties. 

The  two  elements  scandium  and  germanium  were 
discovered  at  a  later  time.  They  also  were  found 
to  fit  into  the  places 'left  for  them  in  his  table  by 
MendelejefT;  and  the  trustworthiness  of  that  arrange- 
ment was  thus  confirmed. 

The  sure  prediction  of  the  properties  of  undiscovered 
elements,  and  the  foreknowledge  of  the  quantitative 
compositions  of  their  compounds  when  none  of  these 
had  been  actually  prepared,  will  always  rank  as  one 
of  the  most  brilliant  pieces  of  work  in  the  domain  of 
chemistry;  for  it  shows  that  trustworthy  conclusions 
of  all  kinds,  in  chemical  matters,  may  be  drawn  from 
those  hypotheses  regarding  the  constitution  of  matter 
which  are  made  use  of  in  chemistry.  In  many  cases 
it  was  only  after  several  years  that  an  opportunity 
occurred  for  demonstrating,  by  the  use  of  the  balance, 
and  therefore  in  a  perfectly  certain  manner,  the  justness 
of  conclusions  that  had  been  drawn  from  these  hypo- 
theses. The  composition  of  gallium  chloride,  which  we 
have  considered,  is  a  case  in  point. 


SYSTEMATIC    ARRANGEMENT    OF 


General 
formulae  of 
(i)  hydrogen 
.  compounds, 
<ii(  salt-forming 
oxides  richest 
in  oxygen. 

Group 
R*O 

I. 

Group  II. 
RA 

Group  III. 
R,03 

Group  IV. 
RH4 

RA 

Series  I. 

Hydrogen 

=     i 



... 

„        2. 

Lithium 

=     7 

Beryllium    = 

9 

Boron 

ii 

Carbon 

=     12 

»     3- 

Sodium 

=  23 

Magnesium  = 

24 

Aluminium  = 

27 

Silicon 

=    28 

»      4- 

Potassium 

=  39 

Calcium       = 

40 

Scandium     = 

44 

Titanium 

=    48 

»     5- 

Copper 

=  63 

Zinc 

65 

Gallium 

70 

Germanium 

=  72 

,,      6. 

Rubidium 

=  85 

Strontium    = 

87 

Yttrium       = 

89 

Zirconium 

=  90 

»»      7- 

Silver 

=  108 

Cadmium    = 

112 

Indium        = 

114 

Tin 

=  119 

„      8. 

Caesium 

=  133 

Barium        = 

137 

Lanthanum  = 

138 

Cerium 

=  139 

„     9- 



,,      10. 

... 

... 



Ytterbium   = 

173 

... 

„    u. 

Gold 

=  197 

Mercury       =  200 

Thallium      = 

204 

Lead 

=  207 

„      12. 

... 

... 





Thorium 

=  232 

Using  the  symbols  in 


Series  I. 

H    =     i 





,,      2.  P  Li  =     7 

Be  =     9 

B    -    ii                 C      =   12 

»      3- 

Na=   23 

Mg=  24 

Al  =  27 

Si     -  28 

,,      4- 

K   =   39 

Ca  =  40 

Sc  =  44 

Ti     =  48 

„      5- 

Cu=  63 

Zn=   65 

Ga  =   70 

Ge    —  72 

„      6. 

Rb=  85 

Sr  =  87 

Yt  =  89 

Zr    =  90 

»      7- 

Ag  =  io8 

Cd  =112 

In  =114 

Sn    =119 

„      8. 

Cs  =133 

Ba  =137 

La  =138 

Ce   =139 

„      9- 





„      10. 

... 



Yb  =  i73 

,,    ii. 

Au  =  i97 

Hg  =  200 

Tl  =204 

Pb    =207 

„      12. 





Th   =232 

R  =  one  atom  of  any  element  in  the  group. 


338 


THE    ELEMENTS    (MENDELFJEFF). 


General 
formulae  of 

Grow/    V. 

Group   VI. 

Group    VII. 

Group   VIIL 

(i)  hydrogen 
compounds. 

RH3 

RH, 

RH 

(ii)  salt-forming 

oxides  richest 
in  oxygen. 

R.A 

RA 

RA 

RA 

Series  I. 

Helium        =     4 



,i        2. 

Nitrogen      =    14 

Oxygen        =    16 

Fluorine       =    19 



»    3. 

»      4- 

Phosphorus  =   31 
Vanadium    =   51 

Sulphur       =   32 
Chromium   =   52 

Chlorine     =  35'5 
Manganese  =   55 

flron          =    56 
\  Nickel      =  58-5 

,»     5- 

Arsenic        =   75 

Selenion       =   79 

Bromine      =   80 

[Cobalt      =    59 

n       6. 

»      7- 

Niobium      =   94 
Antimony    =120 

Molybdenum  =    96 
Tellurium  =126-5 

Iodine          =127 

T  Ruthenium  =  IOI 
\  Rhodium  =  103 
{  Palladium  =106 

„      8. 

Praseodym.  =140 

Neodymium    =  144 



»      9- 

Erbium        =  166 

... 

... 



,,      10. 

Tantalum     =  183 

Tungsten     =184 

... 

rOsmium    =191 

„    u. 

Bismuth       =208 



j  Iridium      =193 
[Platinum  =195 

,,      12. 

Uranium      =240 

place  of  names. 


Series  I. 

„        2. 

»  3- 

n  4- 

„  5- 

„  6. 

»  7- 

,,  8. 

»  9- 

,,  10. 

,,  ii. 

12. 


He=     4 

N    -=    14 

O    =   16 

F    =    19 



P    =  3i 

S    =  32 

Cl  =  35'5 



V    =   51 

Cr  =  52 

Mn=   55 

Jivr-5*. 

As  =  75 

Se  =   79 

Br  =   80           lCo  =  59J 

Nb=  94 

Mo=  96 

fRu  =  ioi 

Sb   =120 

Pr  =140 

Te  =126-5 

Ne  =  144 

I     =127 

(Pd=io6 

Er  =166 

Ta=i83 

W  =184 



fOs  =191 
-  Ir  =  193 

Bi  =208 

... 

... 



U   =240 





339 


/^ 

//  OF  THE 

f   UNIVERSITY 
V 


INDEX. 


ABBREVIATED  names  of  elements, 

21. 

Acetate  of  calcium,  245. 
Acetate  of  lead,  245. 
Acetic  acid.  244. 
Acetone,  187. 
Acetylene,  240,  274. 
Acid  salts,  158. 
Acidity  of  bases,  203. 
Acids,  72,  203. 
Agriculture,  205,  217. 
Air,  analysis  of,  120. 
Air,  pressure  of,  15. 
Air,  weight  of,  134. 
Alabaster,  319. 
Albuminoids,  214. 
Alchemy,  78. 
Alcohol,  241. 
Alcoholic  drinks,  267. 
Aldehyde,  243. 
Alkali,  72. 
Alkali  metals,  316. 
Alkaloids,  256. 
Alloys,  292. 
Alumina,  325. 
Aluminates,  328. 
Aluminium,  323. 
Aluminium  chloride,  323. 
Aluminium  oxide,  324. 
Aluminium  sulphate,  327. 
Alums,  159,  327. 
Alumstone,  326. 
Amalgams,  293. 
Amidobenzene,  253. 


Ammonia,  167,  170. 

Ammonia  water,  168. 

Ammonium,  171. 

Analysis,  12,  145. 

Anhydrides,  117,  202. 

Aniline,  108,  253. 

Animals,  nourishment  of,  204. 

Antimony,  220. 

Antimony  chloride,  221. 

Antimony  hydride,  221. 

Antimony  oxide,  221. 

Antipyrin,  258. 

Argon,  1 20. 

Arsenic,  217. 

Arsenic,  detection  of,  219. 

Arsenic  hydride,  218. 

Arsenic  pentoxide,  219. 

Arsenic  trioxide,  219. 

Artificial  gems,  320. 

Artificial  manures,  211. 

Asbestos,  322. 

Ash  of  plants,  206. 

Asymmetric  carbon  atoms,  259. 

Atomic  weights,  86. 

Atoms,  78,  84. 

Atoms,  arrangement  of,  324. 

BARIUM,  316. 
Bases,  72,  74. 
Basicity  of  acids,  203. 
Bauxite,  324. 
Beer,  267. 
Benzene,  247. 
Benzoic  acid,  54, 97. 


341 


342 


INDEX. 


Beryllium,  321. 

Bitter  salt,  322. 

Blast  furnaces,  290. 

Blasting  gelatin,  187. 

Bleaching  by  chlorine,  48,  318. 

Blendes,  287. 

Blood,  263. 

Blood,  colouring  matter  of,  258. 

Boiling  point,  raising  of,  196. 

Bones,  207. 

Borax,  314. 

Boric  acid,  315. 

Boron,  314. 

Brass,  293. 

Breathing  of  animals,  133,  214. 

Breathing  of  plants,  213. 

Bromide  of  ammonium.  60. 

Bromide  of  potassium,  60. 

Bromide  of  silver,  60. 

Bromide  of  sodium,  60. 

Bromine,  51,  57,  59. 

Bromine,  hydride  of,  76. 

Bronze,  293. 

Bunsen  burner,  281. 

Burnt  gypsum.  319. 

Burnt  lime,  300. 

Butane,  237. 

CADMIUM,  321. 
Caesium,  308. 

Calcination  of  metals,  127. 
Calcining,  289. 
Calcium,  316. 
Calcium  aluminate,  327. 
Calcium  carbide,  276. 
Calcium  hydroxide,  312. 
Calcium  hypochlorite,  232. 
Calcium  oxide,  275. 
Calcium  silicate,  319. 
Calcium  sulphate,  318. 
Candles,  burning  of,  122. 
Cane  sugar,  243. 


Caoutchouc  flasks,  76. 
Carbolic  acid,  254,  274. 
Carbon,  222. 

Carbon,  chemistry  of,  227. 
Carbon,  dioxide  of,  119,  226,  265. 
Carbon,  disulphide  of,  267. 
Carbon  oxide  haemoglobin,  263. 
Carbon,  oxychloride  of,  262. 
Carbonate  of  ammonium,  171. 
Carbonate  of  calcium,  312. 
Carbonate  of  potassium,  305. 
Carbonate  of  sodium,  310. 
Carbonic  acid,  266,  304. 
Carbonic  anhydride,  119. 
Carnallite,  211. 
Caustic  lime,  300. 
Caustic  potash,  299. 
Caustic  soda,  313. 
Cellulose,  121. 
Cements,  317. 
Cements,  hydraulic.  317. 
Cerium  oxide,  283. 
Champagne,  267. 
Charcoal,  222. 
Chemistry,  aim  of,  4. 
Chemistry  and  physics,  i. 
Chemistry,  inorganic,  227. 
Chemistry  of  organised  substances, 

258. 

Chemistry,  organic,  227. 
Chili  saltpetre,  56,  61. 
Chlorate  of  potassium,  1 14,  307. 
Chloride  of  ammonium,  167. 
Chloride  of  antimony,  221. 
Chloride  of  calcium,  318. 
Chloride  of  ethyl,  238. 
Chloride  of  lime,  50,  318. 
Chloride  of  magnesium,  322. 
Chloride  of  nitrogen,  232. 
Chloride  of  potassium,  307. 
Chloride  of  silver,  50. 
Chloride  of  sodium,  67. 


INDEX. 


343 


Chloride  of  sulphur,  148. 
Chlorine,  40,  43,  46. 
Chlorine,  bleaching  by,  48,  318. 
Chlorine,  preparation  of,  43. 
Chlorine  water,  41. 
Chlorobenzene,  250. 
Chloroform,  233. 
Chlorophyll,  213. 
Chromate  of  potassium,  193. 
Chrome  alum,  161. 
Chrysoberyl,  328.  _ 
Citric  acid,  256. 
Coal-gas,  269. 
Coal-tar,  272. 
Cobalt,  298. 
Coins,  293. 
Coke,  271. 
Combustion,  122. 
Common  salt,  67. 
Condensers,  6. 
Coniine,  256. 

Constant    composition    of     com- 
pounds, 52. 
Constitution,  118. 
Copper,  298. 
Copper,  oxide  of,  298. 
Copper,  sulphate  of,  158. 
Copper,  sulphide  of,  287. 
Crucibles,  292. 
Crystalline  form,  53. 
Crystallisation,  54. 
Crystallisation,  water  of,  310. 

DIAMONDS,  222. 
Diamonds,  artificial,  226. 
Dichlorobenzene,  250. 
Distillation,  6. 
Distillation,  dry,  269. 
Distillation,  fractional,  201. 
Distilled  water,  8. 
Double  bonds,  239. 
Double  salts,  159. 


Double  silicates,  319. 
Draught-cupboard,  43. 
Dyeing,  326. 
Dynamite,  187. 

EARTH  metals,  316. 

Efflorescence,  311. 

Electric  furnace,  275. 

Electricity,  33,  65. 

Electrolysis,  33,  301. 

Elements,  19. 

Elements,  modifications  of,  194. 

Elements,  system  of,  329. 

Elements,  tables  of,  22, 88,  338-9. 

Emerald,  328. 

Epsom  salts,  322. 

Equations,  chemical,  23. 

Ethane,  236. 

Ethylalcohol,  242. 

Ethylchloride,  238. 

Ethylene,  273. 

Explosive  gas,  38. 

Explosives,  177. 

FATS,  307. 

Fatty  acids,  307. 

Febrifuges,  257. 

Felspar,  285. 

Fermentation,  267. 

Ferric  sulphate,  295. 

Ferrous  sulphate,  295. 

Filtering,  54. 

Flame,  278. 

Flesh,  204. 

Fluorine,  64. 

Fluorine,  hydride  of,  76. 

Fluorspar,  64,  325. 

Formic  acid,  245. 

Formic  aldehyde,  135. 

Formulae,  calculation  of,  94. 

Formulae,  chemical,  23,  89. 

Freezing  point,  lowering  of,  195. 


344 


INDEX. 


Fruit  sugar,  267. 
Fuchsine,  254. 
Fulminating  mercury,  182. 

GALLIUM,  336. 

Gas  blow-pipe,  284. 

Gas-holder,  114. 

Gas,  incandescent  light,  283. 

Gas-making,  269. 

Gases,  drying  of,  35. 

Gases,  general  behaviour  of,  103. 

Gases,  working  with,  13. 

Germanium,  20. 

Glass,  319. 

Glass  vessels,  boiling  in,  4. 

Glauber's  salt,  314. 

Glucose,  267. 

Glycerin,  307. 

Gold,  287. 

Gold  chloride,  177. 

Gram,  86. 

Granite,  285. 

Graphite,  222. 

Greek  fire,  180. 

Group  reagents,  201. 

Guncotton,  184. 

Gunpowder,  178. 

Gypsum,  209,  319. 

HALOGENS,  151. 
Heat,  129. 

Homologous  series,  242. 
Horn,  216. 
Hydrazine,  172. 
Hydriodic  acid,  76. 
Hydrobromic  acid,  76. 
Hydrocarbons,  230. 
Hydrochloric  acid,  66,  71. 
Hydrocyanic  acid,  234. 
Hydrofluoric  acid,  76. 
Hydrogen,  28,  31,  33,  35,  37. 
Hydrogen  peroxide,  141. 


Hydroxyl  groups,  183. 
Hygroscopic  substances,  139. 
Hypochlorous  acid,  232. 
Hypophosphite  of  potassium,  198. 
Hyposulphite  of  sodium,  151. 
Hypothesis,  atomic,  83. 
Hypothesis,  Prout's,  329. 

INORGANIC  chemistry,  227. 
Iodide  of  potassium,  6l. 
Iodide  of  silver,  64. 
Iodine,  61. 
Iron,  n,  290. 
Iron  alum,  160. 
Iron,  burning  of,  119. 
Iron  chlorides,  293. 
Iron  oxides,  96,  293. 
Iron  rust,  131. 
Iron  sulphates,  295. 
Isobutane,  237. 
Isomerism,  237. 

KAINITE,  212. 
Kieselguhr,  187. 
Kieserite,  322. 
Kipp's  apparatus,  35. 

LACTIC  acids,  261. 
Lead,  298. 
Lead  glass,  320. 
Lead  ores,  287. 
Lead  oxides,  298. 
Lead,  sugar  of,  245. 
Lead,  vessels  of,  76. 
Leaden  chambers,  155. 
Lime,  300. 
Lime,  burnt,  300. 
Lime,  milk  of,  306. 
Lime,  slaked,  134. 
Limestone,  300. 
Lime-water,  133. 
Lithium,  316, 


INDEX. 


345 


Litmus,  73. 


MAGNESIA,  322. 
Magnesia  usta,  322. 
Magnesium  chloride,  322. 
Magnesium  hydroxide,  322. 
Magnesium  oxide,  322. 
Magnesium  sulphate,  322. 
Magnet,  2. 

Magnetic  iron  oxide,  295. 
Majolica,  323. 
Malic  acid,  256. 
Malleable  iron,  290. 
Manganese,  294. 
Manganese  chloride,  296. 
Manganese  oxides,  295. 
Manganic  acid,  296. 
Manganic  sulphate,  295 
Manganous  sulphate,  295. 
Manures,  205. 
Marble,  316. 
Marsh-gas,  230. 
Matches,  191. 
Matches,  safety,  193. 
Meerschaum,  322. 
Mercury,  291. 
Mercury  oxides,  13,  298. 
Metallurgy,  288. 
Metals,  25,  287. 
Metals,  heavy,  287. 
Metals,  light,  287,  298. 
Metals,  oxides  of,  reduction  of,  289. 
Metals,  sulphides  of,  287. 
JMetaphosphoric  acid,  1 1 7,  202. 
Methane,  230,  268. 
Methylalcohol,  241. 
Methylamine,  246. 
Methylaniline,  254. 
Methylchloride,  238. 
Methyl  ether,  242. 
Methylethylpentane.  247. 
.Mica,  285. 


Modifications  of  elements,  194. 
Molecular  weights,  in. 
Molecular  weights,  determination 

of,  in. 

Molecules,  99,  105,  109. 
Morphine,  93,  96. 
Mortar,  316. 
Mother-liquor,  55, 

NAPHTHALENE,  274. 

Nascent  state,  107. 

Natural  science,  78. 

Neon,  136. 

Nickel,  298. 

Nitrate  of  potassium.  173. 

Nitrate  of  silver,  1 76. 

Nitrate  of  sodium,  172. 

Nitrates,  188. 

Nitration,  186. 

Nitre,  172. 

Nitric  acid,  154,  172. 

Nitrification,  215. 

Nitrite  of  ammonium,  165. 

Nitrite  of  potassium,  188. 

Nitrite  of  sodium,  188. 

Nitrites,  1 88. 

Nitrobenzene,  107,  252. 

Nitrocellulose,  184. 

Nitrocompounds,  185,  252. 

Nitrogen,  163. 

Nitrogen,   absorption   by   plants, 

215. 

Nitrogen  oxides,  166,  172. 
Nitroglycerin,  185. 
Nitrogroups,  252. 

OILS,  278. 

Oleic  acid,  307. 

Ores,  288. 

Organic  chemistry,  227. 

Organised  substances,  258. 

Orthophosphoric  acid,  202. 


346 


INDEX. 


Osmium,  27. 
Oxalic  acid,  264. 
Oxidation,  131. 
Oxides,  13,  75,  119. 
Oxygen,  16,  113,  116. 
Oxyhsemoglobin,  258. 
Oxyhydrogen  blow-pipe,  137. 
Oxyhydrogen  gas,  38. 
Ozone,  141,  197. 

PALMITIC  acid,  307. 
Paper,  burning  of,  4,  121. 
Paper,  preparation  of,  313. 
Paraffin,  240. 
Parchment  paper,  157. 
Permanganate  of  potassium,  297. 
Phenanthrene,  192. 
Phenol,  254. 
Phenyl,  254. 
Phlogiston,  125. 
Phosphate  of  ammonium,  204. 
Phosphate  of  calcium,  209. 
Phosphate  of  magnesium,  204. 
Phosphate  of  sodium,  204. 
Phosphates,  203. 
Phosphates  as  manures,  210. 
Phosphoretted  hydrogen,  197. 
Phosphoric  acids,  117,  202. 
Phosphoric  anhydride,  117. 
Phosphorite,  207. 
Phosphorus,  189. 
Phosphorus  chlorides,  199. 
Phosphorus  oxychloride,  200. 
Photography,  152. 
Physics  and  chemistry,  I. 
Pig-iron,  290. 
Plants,  ashes  of,  205. 
Plants,  growth  of,  204,  214. 
Platinum,  287. 
Platinum  chloride,  177. 
Pneumatic  trough,  13. 
Porcelain,  323. 


Potash,  caustic,  306. 
Potash  glass,  308,  320. 
Potash  lye,  305. 
Potash  saltpetre,  172. 
Potashes,  304. 
Potassium,  301,  303. 
Potassium  aluminate,  328. 
Potassium  bromide,  60. 
Potassium  chlorate,  114,  307. 
Potassium  chloride,  307. 
Potassium  hydroxide,  307. 
Potassium  iodide,  61. 
Potassium  salts,  59,  307. 
Powder,  178. 
Powder,  smokeless,  186. 
Precious  stones,  artificial,  320. 
Propane,  237. 
Propane,  synthesis  of,  237. 
Propionic  acid,  245 . 
Propyl,  238. 
Propylalcohol,  242. 
Propylene,  240. 
Prout's  hypothesis,  329. 
Prussian  blue,  234. 
Prussic  acid,  233. 
Purification  of  substances,  54. 
Pyridine,  256,  274. 
Pyrolusite,  297. 
Pyrophosphoric  acid,  202. 

QUARTZ,  285. 
Quinine,  257. 

RADICLES,  227,  242. 
Re-crystallisation,  55. 
Rests,  227,  242. 
Retorts,  16. 
Ring  compounds,  249. 
Roasting  ores,  289. 
Rubidium,  308. 
Rusting,  131. 


INDEX. 


347 


SAFETY  TUBE,  48. 

Salammoniac,  167. 

Salts,  72,  74. 

Salts,  acid,  158. 

Salts,  basic,  159. 

Salts,  nomenclature  of,  117,  347. 

Salts,  preparation  of,  150. 

Sand,  284. 

Saturated  compounds,  262. 

Selenic  acid,  162. 

Selenion,  161. 

Selenuretted  hydrogen,  162. 

Seltzer  water,  267. 

Separating  acid,  176. 

Silicates,  285. 

Silicon,  284. 

Silicon  chloroform,  285. 

Silicon  hydride,  285. 

Silver,  298. 

Soaps,  307. 

Soaps,  hard,  307. 

Soaps,  soft,  307. 

Soda,  309. 

Sound,  2. 

Specific  gravity,  25. 

Spirits,  267. 

Stannic  anhydride,  298. 

Starch,  256. 

Stassfurt  salts,  57. 

Stearic  acid,  307. 

Steel,  290. 

Stoneware,  323. 

Strychnine,  257. 

Sublimation,  62. 

Sugars,  267. 

Sulphate  of  aluminium,  327. 

Sulphate  of  ammonium,  215. 

Sulphate  of  calcium,  209,  318. 

Sulphate  of  magnesium,  322.  t 

Sulphate  of  potassium,  183. 

Sulphate  of  sodium,  314. 

Sulphate  of  zinc,  119. 


Sulphide  of  carbon,  267. 
Sulphide  of  lead,  287. 
Sulphide  of  silver,  146. 
Sulphide  of  zinc,  291. 
Sulphides  of  metals,  147. 
Sulphite  of  sodium,  313. 
Sulphur,  ii,  142. 
Sulphur  dioxide,  149. 
Sulphur,  flowers  of,  143. 
Sulphur  trioxide,  118. 
Sulphuretted  hydrogen,  144. 
Sulphuric  acid,  118,  152,  157. 
Sulphuric  anhydride,  118,  153. 
Sulphurous  acid  gas,  149. 
Sulphurous  anhydride,  150,  155. 
Superphosphates,  211. 
Synthesis,  12. 

TALC,  322. 

Tar,  253. 

Tar  colours,  254. 

Tartaric  acid,  256. 

Telluretted  hydrogen,  162. 

Telluric  acid,  162. 

Tellurium,  161. 

Thiosulphate  of  sodium,  151. 

Thorium  oxide,  283. 

Tin,  298. 

Tin,  calx  of,  127. 

Tin,  oxidation  of,  127. 

Tin  oxide,  298 

Trichloromethane,  233. 

UNSATURATED  compounds,  262. 

VALENCIES  of  atoms,  228. 
Vaseline,  240. 
Vinegar,  244. 
i   Vitriols,  158. 

WASHING- FLASK,  45. 
Washing  gases,  45. 


348 

Water,  4. 

Water,  composition  of,  30,  1 39. 

Water,  distilled,  8. 

Water,  formula  of,  92,  95. 

Water-gas,  5. 

Water,  molecular  weight  of,  in, 

Wax,  77- 

Wine,  243. 


INDEX. 


Wine-vinegar,  244. 
Wood,  313. 
Wood  spirit,  241. 

ZINC,  34,  291. 

Zinc  blende,  287. 

Zinc  oxide,  119,  292,  298. 

Zinc  sulphate,  119. 


Printed  by  Haaell,   Watson,  &  Viney,  Ld.,  London  and  Aylesbury. 


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